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[The Celtic Viking]

No, he's not. Regardless of his origins, he has aligned himself with the Arverni confederation (otherwise he wouldn't be there), of which the Osi do not belong, so you have to be part of one of these options. If Osi lands are later conquered, you can attempt to create a new tribe, though IMO we're too few to have more than 3 tribes. Having many with only 1-2 members is far from optimal IMO.

(Btw, don't take this post the wrong way. You can roleplay him however you want, even to the point of having him as a spy for the Osi tribe or something like that. What I'm saying is that officially he's a part of the Arverni confederacy while the Osi are not, and that's something you should adjust to.)