I agree with everything that was already said and would like to add that the time Oliver would have children would be after the war - so Oliver would know as much as anyone how important it is to stand by people, no matter what their house might be. He might still hold a grudge against Slytherin, but he always respected the other houses, and after the war, he would I think be happy to be able to live to see his children grow up at all...

I always had the feeling that Oliver didn't really have a lot of 'house pride' in particular - rather Quidditch team pride.