Ooh, both are cute. Oscaar is more common and 'safe' I guess but Chayse is quite funky.

Can I ask why you would spell Oscaar with two a's? It's just my personal opinion so please don't take it the wrong way but it seems silly to make a simple name more difficult than it needs to be. Plain old Oscar is just adorable and sounds exactly the same anyway. He'll be forever correcting people regarding the spelling and so will you. Just something to think about!

PS: I can't decide. Both are gorgeous names. Sorry, I was no help at all!