Oscar Velasco is a member of the Khans and has returned to the mountains surrounding the refugee camp at Bitter Springs to exact vengeance against the NCR and anyone else that lives with them. To this end, he has been using his rifle to snipe NCR soldiers and even the civilian refugees being cared for. He has also stolen supplies at night leaving the camp situation at Bitter Springs dangerously untenable for the NCR.
When speaking to Oscar in the cave above Bitter Springs, his voice actor does not pause in between sentences occasionally, as if the voice actor was being rushed.
Hiding then stealth killing him results in no loss of reputation with the Great Khans and also results in good Karma.
Allowing a companion to kill him when he is made hostile through speech results in no Reputation lost or Karma gained.
You will not lose Karma if you fail to convince him and he leaves you no choice but to kill him.
If convinced to return to Red Rock Canyon, he may become a liability if you have Boone in your party, as he remains hostile to First Recon snipers and will turn all other nearby Khans hostile.
If you convince him to return to Red Rock Canyon, you may run into him afterwards while he's attacking the NCR Sharecropper Farms. You cannot engage in conversation with him.
Oscar appears to have unique facial hair, a muttonstache top with a bar brawler bottom.
While wandering, he may walk through Camp Golf, only to be killed by nearby NCR Ranger Veterans there, possibly sniped. He will die if you hang around Camp Golf.
You can convince him to attack Camp McCarran with a small Karma loss.
Oscar is one of the few named non-player characters in the game to be affected by the Sneering Imperialist perk.
He is the only Great Khan that has evil Karma.
After you kill Oscar, if you have Boone as a companion , he will say "Great Khan looks like. Guess he fought at Bitter Springs. Or saw it happen".
If you have Boone as a companion, he will say "Die NCR filth!" and instantly attack you, due to Boone's association with 1st Recon. This will still apply if you encounter out in the wasteland and Boone is with you. However, if Boone is not currently with you while encountering Oscar, it is possible to keep Boone as a companion and Oscar not be hostile towards you.