Feet first into hell!
The ODST is one of the UNSC's most widely known Special Forces organisation. They are equipped with high-tech armour and are one of the most disciplined and skilled soldiers in the UNSC's arsenal. Their nickname "Helljumpers" is well earned, as their main transportation into a battlefield is via SOEIV pod. This pod is quite literally dropped from outer space in order to deliver the ODST troopers the fastest way possible, and since they are relatively small and fast, they are highly unlikely to be destroyed by defensive fire. Because of the high probability that the pods can hit someone on the ground, the ODSTs are always first in the battle, and thus, are always at an immediate disadvantage.
Besides their high-tech armour, they also have a wide variety of weapons to choose from. Each trooper has his or her favourite weapon, and are usually seen with that specific weapon, but they are equipped with most of the weaponry used throughout the UNSC. While not given the same level of resources as the SPARTANs, they are able to complete missions considered too dangerous for normal infantry, using whatever weapons that are available. Specialised weaponry used by the ODST are suppressed weapons, primarily the M7S Caseless Submachine Gun and the M6C/SOCOM. Coupled with their VISR, ODSTs are capable to recognise hostiles in even the darkest areas.
We Are ODST
The way of dropping in soldiers from altitude has been around for a long time, as early as the mid-21st century. It was in World War I that parachutes were first used by the military, and later in the World War II, they were implemented to drop a large amount of soldiers behind enemy lines. But it was not until the Argyre Planitia Campaign that the first use of exoatmospheric drop occurred. Which showed that the concept worked, and paved the way for a fully established ODST force at the conclusion of the Rain Forest Wars and the Jovian Moons campaign at the end of the 22nd century.
When war broke out once again in 2490, the ODST stood on the sides of the Unified Earth Government to fight the Insurrection. The ODSTs were becoming more and more crucial for the UEG, a problem which resulted in ONI to green-light a program which could replace the ODSTs as the top-fighters. This program was done in absolute secrecy, and the ODSTs did not know much about it.
But one incident aboard the UNSC Atlas changed this. While a young John-117 was working out in the ship's gym, he removed a pin from the bench press, and forgot to put it back before leaving the apparatus. Sometime later, when a couple of ODST soldiers came to train on the bench press, the weights fell which almost crushed one of the soldier's feet. A fight ensued and John killed three of the ODSTs and severely injured the other. Ever since then, relations between these 'SPARTANs' and the ODSTs have been tensed and uneasy.
You know the music; time to dance
And then, in 2525, it finally happened, we made contact with aliens. The entire UNSC moved into a full alert start to battle this new foe, this 'Covenant'; this marked a change in objectives for the ODSTs, who up until that point had been fighting the Insurrectionists. Even though there had been sporadic fights between the Humans and the Covenant, the Battle of Hat Yai is considered to be one of the first battle in the Human-Covenant war; it was the first ODST-Covenant encounter.
In the following years, the ODST corps took great casualties in the war. With massive losses at Circinius IV, the Battle of Skopje, the Battle of Sigma Octanus IV and the Battle of Tribute. But even with these losses, the ODSTs prevailed and continued on. However, it was not until the Battle of Reach, that truly tremendous casualties were felt. The battle raged on for a week with 750 fully equipped Covenant warships and 152 UNSC warships on the other side, both with very heavy ground assets. And even though the Covenant eventually won the battle, resulting in countless ODST casualties, they did not prevent the SPARTAN-II and -III team Noble Team from delivering a package to the UNSC Pillar of Autumn, which would turn the tide of the war.
But it wasn't until a small Covenant fleet jumped into the Sol system and came towards Earth that the first steps towards the end of the war were taken. ODST combat teams, along with the Marines, began engaging Covenant forces on the Orbital Defence Platforms Cairo, Athens, and Malta. Despite the ODSTs' valiant efforts, Malta and Athens were destroyed. As the Covenant ship carrying the Prophet of Regret landed over the African mega-city of Mombasa, teams of ODSTs were deployed over the city from orbit in an attempt to board the carrier; before the teams could reach the target, however, the carrier jumped into Slipspace; only a single squad managed to land safely.
After the Covenant fleet abruptly fled, Mombasa in particular was still full of Covenant ground troops; mostly Jiralhanae, Unggoy and Kig-Yar. Since there were a lot of ODST personnel present on Earth at the time, this was one of the largest campaigns they fought in the entire war. After a returning Covenant fleet opened The Artifact near Mombasa, the 11th Marine Force Reconnaissance/ODST, stationed aboard UNSC Forward Unto Dawn, fought alongside John-117, during Operation: BLIND FAITH. Men and women from this battalion were also some of the first humans to travel outside the Milky Way galaxy, during the Battle of Installation 00. During the last battle of the Ark campaign - the last battle of the war - ODSTs were again airlifted into combat. This time, they punched holes in Covenant armor and artillery positions to allow John-117 and Thel 'Vadam to eliminate the Prophet of Truth.
When the SPARTAN-IV program was begun, many of the most decorated ODSTs enrolled in the program and received their augmentations. This led to a split in branches, as the SPARTANs now were moved into their own respective branch, separate from the UNSC Navy they had once belonged. Some of the ODSTs that became SPARTAN-IVs were Edward Buck, Kojo Agu, Paul DeMarco and Sarah Palmer.