Much of Osis' early career is not recorded. He had fought on Luthien in 3052 during Operation Revival. In 3057 he held the rank of Star Colonel and was the Aide de Camp to Khan Lincoln Osis. In this position he had accompanied the Khan to several Grand Council meetings picking up a great deal of knowledge and political acumen.
He had tested to the rank of Galaxy Commander, but the lack of an open position prevented him from obtaining a command and he retained his lower rank while awaiting an opportunity. It says something about his character that he did not enact a Trial to obtain the rightful command from an incumbent commander. However, by April 3058 Lincoln Osis was in a position to offer Devon the command he had earned with the newly raised Tau Galaxy. He traveled to their cantonment on Wayside V and formally took command in June 3058.
The Battle for Wayside V
On July 3rd 3058 Stirling's Fusiliers regiment of the Northwind Highlanders jumped into a pirate point in the Wayside and issued a Batchall to the Jaguar ground forces Devon Osis answered this challenge, then called his Star Colonels together to bid for the right to engage the mercenaries.
Osis' allowed his commanders a free hand in fighting the mercenaries, but seemed oblivious to the fact that he was allowing his command to be whittled down by small encounters were handfuls of warriors were sent against superior numbers of Highlanders that led the warriors of Tau on a chase across the planet. By the 8th August the 250th Assault Cluster had caught the mercenaries and engaged them in combat. As they did they were hailed Star Colonel Santin West with a Batchall. Thinking he faced a single Cluster Devon Osis bid his entire command in the hope of quickly crushing the Nova Cats. West crushed his ideas when he revealed he had temporary command of two Nova Cat Clusters; the 179th Striker Cluster and 246th Battle Cluster. 
The Nova Cats landed a kilometer from Tau's Clusters but quickly advanced to almost point blank range. Galaxy Commander Osis met Santin West in the midst of the furious battle and challenged him to single combat, however just as Osis was about to deliver a death-shot from his Dire Wolfs ER PPCs, Major Loren Jaffray of Stirling's Fusiliers, who had been masquerading as a Jaguar warrior shot and killed Osis.
Devon's description is not given, however Impetus of War noted he was of the same Bloodhouse as Thibideau Osis, one of his Star Colonels, and that they shared the same genetic heritage. Thibideau was described as tall, blond and blue-eyed, so it is likely Devon had a similar appearance. During his dealing with ilKhan Osis the two are never mentioned as being from the same bloodhouse.
- Impetus of War, p. 83
- Invading Clans, p. 70
- Impetus of War, p. 26 and 64
- Impetus of War, p. 74-87
- Northwind Highlanders (scenario pack), p. 32-4
- Impetus of War, p. 269-70
- Impetus of War, p. 274
- Impetus of War, p. 285-6