I suppose that's true...it's not like Ossanlin was cut off from the Homeworld during Salem's time there. He'd have likely heard news. And if Salem is going to be familiar/friendly with Raigar, it's like that the War Prince would've kept Ossanlin apprised of the unique situation.
Yeah... I'm still debating just how close I want Salem and Raigar to have been, in no small part due to the fact that I don't trust myself to write your character properly, but they'll definitely have known each other and have been friendly. There's no particular
reason I can think of that Raigar would keep Ossanlin apprised of Salem's progress or anything, but having a human on the homeworld alone would be enough to warrant at least the occasional passing comment. Not to mention the rumors and the (probably mostly derisive) comments from other Andalites. Salem's position on the homeworld was never exactly a secret, but amidst Andalite xenophobia, it was likely a source of an undercurrent of shame that rendered talking about him too much quasi-taboo.
At some point soon, I'm planning to run through my old timeline of Salem and line it up, blow-by-blow, with the Rebellion timeline. If I recall correctly, Salem would have spent somewhere between three and five years on the Andalite world, ending shortly before his initial introduction in the GESB. I actually need to re-work a lot of Salem's timeline right in that area to coincide better with the Rebellion and Andalites.
Salem, of course, would have known Ossanlin's growing reputation from the media feeds/propaganda/whatever you want to call it. Raigar strikes me as the type who would seldom discuss a former student by name, so if Ossanlin ever had come up in conversation with Raigar, I'm not sure Salem would have connected those comments with the Andalite, even now. Depending on how the timing works out, Salem may have run across Siruvan as well. How long after Ossanlin would Siruvan have graduated?