Star Trek Canon Special Note
Not everything listed in the origin is Canon. That is because only television shows and movies are considered canon for Star Trek. This being a comic website there are many things here that are not canon. So unless it is contradicted in film it should be listed as part of the character biography.
Odo is a member of an alien race known as "The Founders." The founders originated on their home planet in the Gamma Quadrant of space. They are an alien race who possess the ability to shape-shift into any object or individual that they come into physical contact with. However, each member must work on their individual shape-shifiing abilities. This alien race is served and worshiped by other aliens known as The Dominion.
Little was initially known of Odo's true origin. However, through the course of time, Odo managed to arrive in the Alpha Quadrant. A group of space travelers found him floating in space. At the time, Odo was in a somewhat liquid physical state. Unknown at the time, change-lings spent their origin and most of their lives in a gelatinous state - a state between molds. The travelers thought it best to turn the matter over to a scientist on the planet Bajor.
At the time, Odo was sent to Bajor the planet was occupied by an alien race known as the Cardassians. They used Bajorans is forced labor camps and comfort stations. So tremendous pressure was put on the Bajorian scientist to identify the sample. Odo's very name means unknown sample. Through the work of a dedicated Bajoran scientist, it was discovered that this "matter" was intelligent life. Working with the matter, the scientist began to train it. Odo was taught to interact with people and develop his shape-shifting ability.
Orbiting the Bajoran home-world was a Cardassian space station known as Terac' Nor. By this time Odo had developed the ability to create and maintain a particular form. The Cardassian commander Dukat therefore brought Odo to the station to enlist his help in solving a crime. When Odo arrived, it was thought that his abilities would help in the area of security. Even though the crime would not be solved until long after the Cardassians had abandoned the station his experience would prove very useful in times to come.
Finally, the Cardassians withdrew their forces from Bajor. As the Cardassians abandoned the space station, they left it in shambles. That is when the United Federation of Planets intervened on Bajor's behalf. They arrived and resurrected the station, renaming it Deep Space Nine. Odo having always been looked on as a fair man was accepted into the Bajoran militia and stayed on as security chief. By this time, he was a seasoned security constable. Because the station would be undergoing a rough transition, criminal activity would be at an all-time high. Now, more than ever, Odo's expertise was needed.
The character of Odo was created for the Star Trek: Deep Space Nine television show. The whole concept of the show was an old western town and Odo was meant to be the sheriff. They also wanted him to have a way about him like the actor Clint Eastwood did in many of the spaghetti westerns of his era.
During Odo's time working along side the Federation he carried on pretty much as he always did. He did however have a few more restrictions than he had in the past. One other thing that remained the same was Quark. Quark was the nefarious bartender who was the yin to Odo's yang. The two were always at odds but have an underlining affection for each other.
This continued on for Odo until the interactions with the Dominion. The Dominion was lead by his people. Forcing him to not only live separately from his people but to actively fight against them. As the Federation was a war with the Dominion Odo was even put in a situation were he was forced to kill another changeling. This had never been done. As a punishment Odo was captured and turned into a "solid". In other words he was a human with no ability to shape shift.
Rejected by his people Odo found new life in finding an infant changeling. Odo tried to raise the changeling the best he could but was unable to lick with it as was his people's custom. Unfortunately the changeling was ill and dyeing so it never grew to it's potential but it did link with him giving him back his shape shifting abilities.
The war went on for Odo and the cost was great for both sides. All changelings developed a virus that was slowly killing all of them. Even Odo was ill and it was found out that Odo was infected by the Federation to spread the virus to his own people. Then the Federation refused to give Julian Bashir the cure to save Odo's life for fear the enemy would get it as well. Julien was however able to steal the cure from the Federation secret agency.
The war went on and only ended when Odo offered to cure the other changelings. He also told them that he would go back to the planet of the Changelings and live there for an undetermined time. This was hardest on his love Kira Nerys. Odo had been in love with Kira for over seven years and she had fallen in love with him only recently. Having him leave meant he would no longer see her and he did not know when or to be certain if he would see her again.