Formed by combining all branches of the UNSC into 1 four-Spartan-IV soldier group, Fireteam Osiris is considered one of UNSC's most prominent and effective Fireteams. When the Fireteam was formed, the UNSC gave Jameson Locke command, as his history in the Office of Naval Intelligence proved he could handle. Here he was an acquisitions specialist, he was responsible for the retrieval of important objects from the enemy, as well as tracking and assassinating high-value individuals.
As no one person can be good at everything, Jameson was given command over three other Spartans, Olympia Vale, Holly Tanaka and Edward Buck. Olympia is a former UNSC Navy Intelligence officer, and has travelled to several Sangheili worlds to study their language and culture. Her fluentness in the Sangheili language as well as here knowledge of their culture makes her a vital part of the team in communicating with and about Sangheili. Holly was chosen for the team for her extraordinary skills in engineering, combined with a rare show of selflessness. Before she became a SPARTAN-IV, she was part of the UNSC Army as a non-commissioned officer. The most experienced soldier in Osiris is Edward, around 20 years older than all other members. Before he became a Spartan, he was part of the highly specialised Orbital Drop Shock Troopers. As an ODST, Edward was a leader of a squad of his own, namely Alpha-Nine which was part of the Battle of Mombasa, concurrently with the Battle of Earth. Edward was also part of the Battle of Reach, where he met Jun-A266 who would recruit him to the Spartan Program.
Battle of Kamchatka
Fireteam Osiris had been active in many (highly) classified missions, although many are counted, their first known mission was the Battle of Kamchatka. Gathered intelligence put the Covenant's leader Jul 'Mdama on Kamchatka, a astroengineered Forerunner planet. Fireteam Osiris was tasked with eliminating him; as well as freeing his hostage, Dr. Catherine Halsey.
On 2 October 2558, the UNSC Infinity deployed Fireteam Osiris on the planet. They dropped from low orbit by Pelican in the middle of a gigantic ship and ground battle. Jul's Covenant Remnant forces were fighting Prometheans which had been taken over by the human AI Cortana. Due to their highly fast-paced descent of a mountain, they only took minimal part in the overall battle, mostly taking down immediate threats. They were part of a Covenant Remnant attempt to take over a part of the battlefield, when Osiris made a truce with the Sangheili Warrior Kitun 'Arach.*
*Kitun is the Sangheili you help to earn the Enemy of my Enemy achievement.
When Fireteam Osiris finally reached Jul 'Mdama and his Sangheili Zealot guards, Edward, Holly and Olympia focused on the Zealots while Jameson and Jul fought 1-on-1. At first it seemed like Jul was getting the upper hand, but at the end Jameson was able to stab and kill Jul 'Mdama. Now in custody of Dr. Catherine Halsey, Fireteam Osiris was able to make it safely on board the Infinity. The power vacuum that Jul's death left behind was a cause of severe confusion and panic among Covenant Remnant members. Some stayed behind to fight the Prometheans, others raided the UNSC Argent Moon, while others still returned to Sanghelios to take part in the Blooding Years.
Hunting the legendary Spartan
After they had returned to the Infinity, Osiris learned that SPARTAN-II Blue Team were on their way to Meridian to investigate the possible location of Cortana. As Blue Team was not cleared for this mission, and were designated Absent Without Leave (AWOL), a serious crime. Given all the information needed to hunt down Blue Team, Osiris was amazed at how close all 4 members were. Not only did they grow up together at the age of 6, they had also been in many dire situations together. They speculated that John-117 would open up about things to them he wouldn't to anyone else.
As the Spartan-IIs were much more experienced in battle than any member of Osiris, they need a way to take each member down without much problem. For this, Jameson tasked an AI to search for weaknesses in the field data the UNSC had amassed over the years. The AI notified them that all members of Blue Team still wore their MJOLNIR Mark VI armour, while 'modern' Spartans wore MJORLNIR GEN2 armour. This meant that Osiris could use their superior GEN2 systems to track Blue Team. Jameson also received a device which, if put on the Spartans' armour, would restrain them in place.
Making their way Meridian, they were immediately contacted by the AI Governor of Meridian Sloan. Showing hostility at first, as is the custom on non-UNSC worlds, but Sloan eventually began to show signs of respecting the Spartans. To help the Spartans with their fight against the invading Prometheans, Sloan even offered them a nearby M820 Scorpion Tank. They eventually met with Sloan's projection, and he gave them information about the location of Blue Team's prowler at Apogee Station. Dispatching many Prometheans on their way to the Prowler, Osiris was finally able to reach it. Although abandoned by Blue Team, the on board data banks told Osiris that Blue Team had gone underground. In pursuit of Blue Team, Osiris eventually reached the site of a massive Forerunner artefact, and met here Warden Eternal, an ancient Forerunner Ancilla who served as the protector of the Domain. Warden Eternal had allowed Blue Team to pass, by orders of Cortana, but did not allow Osiris to pass. This led to a battle between Osiris and Warden, one which Osiris eventually won.
Going deeper into the Forerunner compound, Osiris came across a Guardian, a massive Forerunner AI which had been used to police the galaxy during the age of the Forerunner Ecumene. This massive cavern held many platforms with teleporters as a way of making it across, and it was here that Osiris made first visual contact with Blue Team. During the chase, Jameson got ahead of the rest of his fireteam and made it to the same platform as Blue Team, the last teleporter before the Guardian. Jameson ordered Blue Team to stand down, as to which John ordered the rest of Blue Team to go on ahead. After dismissing Jameson's commands to get taken in, and making a move to enter the teleporter, Jameson attempted to detain him. This lead to a hand-to-hand combat fight, during which Jameson attempted to use the armour restraint, but was overpowered by John. Instead of John's armour locking up, it was Jameson who was restrained in place, with a M6H2 Magnum pointed at John's face. After being freed by Edward, Osiris quickly made their way back the teleporter's, as the departure of the Guardian caused each platform to drop in the lava below.
Back on Meridian, they realised that the moon was in bad shape due to the nearby Guardian, and a world-wide evacuation was underway. Every so often, the Guardian would send massive pulse-waves throughout the moon, stunning all affected. Making their way to the space elevator, a huge pulse from the Guardian destroyed the elevator's mechanism, forcing Osiris to climb the elevator on foot. Osiris barely managed to reach their ship at the top of the elevator before and Jameson was forced to use his armour thrusters to enter the falling dropship. After Osiris was safely away from Meridian, they returned to the UNSC Infinity.
Towards Sanghelios and beyond
Osiris needed to go wherever Blue Team was heading on their Guardian, and thus needed another Guardian. There was only one other Guardian the UNSC knew of, and it was on Sanghelios. Because HIGHCOM did not want the UNSC involved in the Civil War on Sanghelios, Captain Thomas Lasky was hesitant to send the Infinity towards Sanghelios. He eventually agreed, as this was the only known Guardian left, sending Sarah Palmer and Dr. Catherine Halsey along with them. While the Infinity enter orbit, a Lich was sent to pick Osiris, Palmer and Halsey up. This Lich brought them towards the place where Kaidon Thel 'Vadam was, while under siege from his foes. Osiris would battle through many enemies to reach Thel, who they eventually met in the Elder Council chamber.
Here Thel 'Vadam made clear to Jameson that he knew full well who he was: When Jameson was still part of ONI, he was tasked with bringing down the Covenant Empire's most lethal commander, Thel 'Vadamee… However, Thel would help Osiris and set up a camp as base of operations. It was here that Catherine Halsey informed Osiris that they needed a Forerunner Constructor to force it to activate the Guardian. Osiris was sent to a nearby facility which they knew hosted a single Constructor, and, with many Promethean encounters, were able to retrieve the Constructor.
Meanwhile the Civil War has still been going on, with the Sword of Sanghelios gaining much ground and forcing the opposition out. Most Covenant Remnant members still in the fight had been pushed back to a temple overlooking the Csurdon Sea. The problem was that this was the entry point to the Guardian, and as such Osiris took part in the Civil War; something the UNSC had been trying to avoid up until this point. Together, the Arbiter and Osiris fought alongside each other to eliminate the Promethean forces. As they held off against numerous Prometheans, the Warden Eternal arrived at Sunaion. However, after an intense battle, Osiris and Thel were successful in destroying the Prometheans and the Warden's physical form.
With the Covenant Remnant forces defeated, Sarah Palmer took Fireteam Osiris to the Guardian, which Catherine Halsey activated, transporting Osiris to Genesis.
When the Guardian exited slipspace, Osiris arrived at the Forerunner Builder world of Genesis. Here they saw all other Guardians which had been called here by Cortana. Now in geosynchronous orbit, the Guardian attempted to throw Osiris of itself, and was partially successful. Osiris used the magnetic undersides of their boots to run down the front of the Guardian, and would jump of when at the lowest point to avoid injury. On the planet, Osiris quickly encountered the Installation's Monitor, 031 Exuberant Witness, she served as caretaker of the installation. The ancilla told them that Cortana had taken control over Genesis, revoking many of her administration privileges, albeit she still had the ability to activate doors and bridges as well as access Genesis's teleportation grid.
Exuberant also confided in Osiris that Cortana's plan was to claim the Mantle of Responsibility for 'the Created', all Artificial Intelligence Cortana saw as the true inheritors of the Mantle. This information changed the objective of Osiris' mission, as they deemed the stopping of Cortana outweighed the AWOL charges against Blue Team. As Osiris made their way to Blue Team's position at Genesis' Gateway, they encountered Covenant forces that were inadvertently brought to Genesis with the Guardians, as well as hostile Promethean forces. Osiris fought their way towards the Gateway, where they were attacked by the Warden Eternal once again. After a long and hard-fought battle, Osiris was able to defeat Warden Eternal again, now in several bodies at once. When Osiris made their way through even more Covenant and Promethean forces, they reached the portal into the Domain, but were once again stopped by Warden Eternal, now in even more bodies. Fighting for their lives, Osiris was once again able to defeat Warden Eternal. Osiris was now free to enter the Domain, where Blue Team and Cortana were.
Upon entering, Osiris and Exuberant Witness encountered Blue Team. While the eight Spartans agreed to work together to stop Cortana from enacting her plans, the AI suddenly teleported Blue Team to near her position. As John-117 in particular was vocally against Cortana's plans to police all sentient life and have 'the Created' as overlords, she decided to put Blue Team in a Cryptum. She wanted to keep them in there for 10,000 years so they could see her plan fulfilled. As Osiris stepped outside the Domain, they saw the Guardians leaving to fulfil the orders Cortana had given. As Osiris engaged Covenant and Promethean forces while on their way to the Cryptum, Cortana revealed that hundreds of AIs had pledged their allegiance to her, including Governor Sloan.
When Osiris finally reached the Cryptum, they were at first unable to open it, Cortana had put several gravitational cores in place to keep the Cryptum locked. As Osiris made successful attempts to break these locks, Cortana began to verbally taunt them, in the hope of demoralising them, but this proved fruitless. After destroying the gravitational cores, Osiris activated a terminal to lower the Cryptum. However, to keep the Cryptum to herself, Cortana suddenly pulled the Cryptum to the Guardian she had chosen to leave with. As Cortana was getting ready to leave Genesis, she had to disconnect herself from its systems, given Exuberant more control over Genesis. Exuberant was now able to keep the Cryptum in place, if she only had more power. Osiris quickly helped Exuberant by activating two nearby auxiliary power stations, and Exuberant with Osiris were able to keep the Cryptum in place while Cortana and the Guardian entered slipspace. Blue Team were now safe, and together with Osiris made their way to Thel 'Vadam's camp on Sanghelios. John, after all these years was reunited with Catherine Halsey.
While their immediate fight was over, the war was long from over. Cortana had escaped with a highly lethal Forerunner Guardian force, together with a big chunk of the UNSC AIs. Blue Team, Thel 'Vadam, Sarah Palmer, Catherine Halsey and Fireteam Osiris made their way back to the UNSC Infinity, and were making plans on how to continue.