The Founders would originally claim that this was to gather knowledge of the rest of the galaxy by aborbing the experiences of The Hundred upon their return. It was later discovered the Hundred were, in fact, bait to lure the being known as the Progenitor, the entity that created the Great Link enmass and then left for parts unknown, to return. The Founders considered the Progenitor to be the one true God of the galaxy.
Odo was found in 2356 in his gelatounous state in the Denorios Belt, and brought to the Cardassian occupied planet of Bajor, where he was studied and later tutored by Dr. Mora Pol. Odo would later leave the lab that was the only home he knew, and relocate to the orbiting space station Terok Nor.
Odo's name was originally Odo'ital, this was given to him by Dr. Mora before it was discovered that Odo was a sentent being. the name translates directly as "Unknown Sample". Later on, Bajoran scientists studying Odo would change his name to the more Bajoran sounding Odo Ital, eventually Odo shortened it to Odo.
Chief of Security
Odo was recruited by Gul Skrain Dukat, the Cardassian Prefect of Bajor, to serve as the stations Chief of security in the year 2365. He would serve in the position past the Cardassian withdrawl and well into the tenure of Starfleet, until the end of the Dominion War in 2375.
Deep Space Nine
After the Cardassian withdrawl in 2369, the Bajoran Provisional Government requested relief efforts from the United Federation of Planets, and gave occupancy of the station to Starfleet, who rechristened it Deep Space Nine.
At first, Odo didn't think he was going to like the station's new commanding officer, Commander Benjamin Sisko, but after seeing how Sisko handled Odo's nemesis, the Ferengi bartender Quark, Odo warmed up to him. (DS9 novelization: Emissary)
Odo first met a being like himself several weeks after the arrival of Starfleet when the gelatinous shape-shifting serial killer Meta stalked DS9.
Odo and Meta fought outside the station and drifted through space towards the Bajoran wormhole, which was undergoing violent subspace compression, destroying anything that entered. The Ops crew managed to beam Odo back to the station, but Meta was drawn into the wormhole and destroyed.
It is unknown whether Meta was an insane Founder, a member of the Hundred, a member of another link spawned by the Progenitor, or something else entirely. The Changeling credo that no Changeling shall harm another is intact as Meta essentially caused his own demise.(DS9 novel: The Siege)
Following an outbreak of Bajoran flu aboard the station in 2370, Odo was vaccinated along with the rest of the station personnel. However, unlike the other personnel, Odo reacted badly to the the vaccination and began to experience memory loss and bouts of extreme paranoia. After being chased around the station for several hours, in which time he attacked several personnel, Odo was forced to head to the infirmary where he reverted to his gelatinous state. As he rested, Odo's memories returned and the effects of the vaccine wore off. (DS9 comic: Lapse)
Later that year, Odo assisted Starfleet in preventing an assassination of Klingon Chancellor Gowron by former Starfleet officer and Maquis Thomas Riker, who was actually a pawn of the Romulan Sela. (TNG novel: Triangle: Imzadi II)
In 2375, Odo returned to the Great Link with the cure to a virus that was decimating the Changeling population in exchange for the Dominion's surrender in their war against the powers of the Alpha Quadrant. He left behind the woman he loved, Colonel Kira Nerys. (DS9 novelization: What You Leave Behind)
The Great Link
Soon after arriving at the link, Odo confirmed that there was indeed genetic material on file for the cloned Vorta Weyoun, despite previous claims. Odo had a new Weyoun cloned to act as his personal Vorta assistant.
Three months after rejoining the Great Link, Odo assigned Taran'atar, a Jem'Hadar soldier that was not addicted to the drug Ketracel White, to travel to Deep Space Nine and act as an observer of the species of the Alpha Quadrant. Odo's hope was that the Jem'Hadar could eventually learn to be more than blood-thirsty soldiers. (DS9 novel: Avatar book two)
Soon after dispatching Taran'atar to DS9, rumors reached the Great Link of a holy woman speaking of the Prophets and of healing. The rumors also said that this woman had had contact with a mysterious race known only as the Ascendants, a dangerous race that the Link knew very little about.
Believeing this woman to be the lost Bajoran Kai Opaka Sulan, Odo disguised himself as a Trelian woman named Wex and set out in search of her. Odo eventually found both Opaka, and Jake Sisko, who was on his own adventure in the Gamma Quadrant.
Opaka's knowledge of the continued existance of the Ascendants was important information for the Dominion, but Odo couldn't return to the Link without first seeing Opaka and Jake back home to Bajor.
Upon arriving back at the station, Odo quickly found himself in the middle of the Parasite crisis. Reuniting with his love Kira, Odo assisted in the liberation of the monastery at Ashalla, which was the stronghold of the alien parasites.
Odo and fellow member of the Hundred, Laas, learned the truth as to why they were sent out into the galaxy as infants at the end of the year 2376, from a Changeling that Odo named Indurane, Bajoran for 'ancient'.
Indurane explained that the Great Link and all Changelings were created enmass by a god-like being known as the Progenitor. There were no 'infant' Changelings, only Changelings that had not separated from the Link and begun to learn. This was why the Changelings had such restrictions on harming one another-- with each Changeling death, the species was brought that much closer to extinction.
The Hundred had been sent out to attract the attention of the Progenitor and lure it back to the Link so it could replenish the species.
For a brief time, it appeared as if the Changelings had succeeded and lured the Progenitor back when a nova appeared in the sky in the place where the Founders believed the Progenitor would return. But when a party of Changelings, including Odo, inspected the site, they found only the corpse of a gigantic Changeling, perhaps even the Progenitor itself, killed by the radiation of the nova.
Several days later, the Great Link dissolved in what appeared to be a mass suicide, as all of the Changelings that had been hoping for the Progenitor's return seemingly gave into their sorrow.
This left only Odo, Laas, and perhaps a few other members of the Hundred running the entirety of the Dominion. As of this writing, the future of the Dominion is in question. (DS9 novel: WoDS9 volume three: "The Dominion: Olympus Descending")