Olive oil or butter? Both have an amalgamating property, bringing everything together texturally, that is always used to ‘finish’ risotto.
Many people mistakenly think that butter, and lots of it, is required as the finish, to make risotto creamy. Olive oil at the end adds a nice complexity that does not alter the essential flavor of the risotto: it is, in my opinion, a cleaner finish. I like using olive oil as a finish with fish risotto and some vegetable risottos, because it leaves the pristine flavor of the fish and vegetables clear and vibrant.
Butter, on the other hand, is a marvelous amalgamator: it makes the risotto even creamier, and obviously, buttery. I use it with all meat, all mushroom, and some vegetable risottos. The butter makes it rich and creamy, magnifies and to some extent alters the flavor. This can be desirable and there are many risotti where I love to use it. For instance, butter has the effect in tomato sauces of balancing the acidity, but it changes the taste in a way that olive oil doesn’t.