Hello. I was wondering if the DOHC heads off my outback will fit a 2.5 block that originally was SOHC? Thanks for your time.

Yes. Direct swap no problem.

I think the combustion chamber is smaller on the dohc, so you'd have somewhat higher compression ratio after the swap.

Nope just a difference in the pistons themselves but compatible all the same. SOHC pistons are higher compression.

Your new compression ratio will depend on the gasket you use. I suggest a cometic 60 thousands. Should be around 10.6:1. should be good on 89 octane.