hi sarah, just browsing your profile and your posts
i really don't think it would be wise for him to go in with your 5" oscar, oscar's don't do well in pairs unless they are proven breeding pairs, you can't just put 2 together and expect them to get on (unfortunately).
if you want to keep more than 1 together as adults you need a minimum of 6 and for that you're going to need a significantly bigger tank than the one you have.
so sorry for the time being i don't think it's suitable for him. thanks for the offer though.