Oss originally trained under the Neti Jedi Master Garnoo; however his training was cut short as Master Garnoo was nearing the end of his life. Before he passed into the Force however, he introduced Oss to Jedi Master Thon of Ambria, and Thon, recognizing Oss' strength in the Force, agreed to take Oss on as his apprentice once Master Garnoo became one with the Force. Oss traveled to the planet of Ambria with Master Thon to continue his training. Oss excelled at his studies very quickly, and after five years had already exceeded Thon's expectations. During his training on Ambria, Oss and Master Thon were eventually joined by Nomi Sunrider who had also come to Thon for training in the Jedi way.
But the dangerous Hutt, Bogga, had followed Nomi to Ambria, and he, along with his thugs intended to attack Nomi and Thon's compound. Once they had arrived on planet and threatened the camp, Oss took up his lightsaber for defense against the attackers. However, the thugs were too numerous in number and great in size, and were able to overpower the young Jedi. However, before they could inflict any serious harm, Oss was rescued by Master Thon who drove away Bogga and his henchmen. In the months that followed, Oss repaired Nomi's damaged protocol droid and continued to train with Master Thon and Nomi Sunrider who began her rudimentary introductory training.
Oss could not spend all his time in training however, and soon the events of the galaxy called upon Oss to answer to them. The Jedi Knight Tott Doneeta came to Ambria to enlist Oss' help in confronting the growing evil on Onderon. With his Master's approval, Oss joined the Twi'lek Jedi to travel to the other side of the galaxy to put down the Freedon Nadd Uprising. Oss and Tott joined the rest of the Jedi, Master Arca Jeth, and Jedi brothers Ulic and Cay Qel-Droma, just after the Darksider Queen Amanoa was defeated and the new rulers crowned on Onderon. Oss was introduced to the new rulers Queen Galia and Oron Kira and expressed his eagerness in helping with the work of extinguishing all the darkness from the walled city of Iziz on Onderon.
It wasn't soon after that the Dark Side Naddists attacked led by the armored warrior Warb Null. Oss and his fellow Jedi fought against the Darksiders and were successful in preventing their attempt to reclaim Freedon Nadd's remains. Oss and the Jedi were soon led to the underground lair of the leader of the Naddists, King Ommin. During the meeting the spirit of Freedon Nadd appeared and captured Master Arca in a field of Dark Side energies. Determined to rescue Master Arca, Oss and the Jedi called for the help of Modon Kira and the Beastriders. However, Naddists trapped Oss, the Jedi, and the Beastriders, in Modon Kira's forest citadel.