Osian is a truly beautiful name! But then I'm biased because our DS is Ossian (Scottish version). We've had really good responses from people. Irish people often know the name, Scottish and Welsh less so, and occasionally English people have heard of it. But it's such beautiful sounding name, people really like it, and I don't think it matters that's it's unusual. (But then I like unusual names!)
It has a lovely meaning too. Osian means 'little deer'. He appears in Celtic mythology as the son of Finn McCool, a bard/poet and member of the Fianna tribe, and also the lover of the fairy Nia (Niamh), who he goes to Tir Na N-og with (the land of the forever-young).
Finn was at the top of my list for a while, until I realised just how popular it is these days. I remember someone on a mnet thread at the time said that out of 25 children in her DC's nursey class, 5 of them were called some version of Finn! And it's really lucky we didn't go for it in the end, as DS's cousin was born 3 weeks earlier, and named Findlay.
I'm not too fussed on Oliver. (A bit too posh English perhaps ). Ollie is a nice NN though.
I really like Ruby and Seren. Seren was on my list originally, but a friend got there first with her DD (and bump was a DS in the end anyway). I do wonder if it's getting a bit popular now, although I don't know if that matters to you or not?
Ruby is just a lovely name .