I like when my partner uses a smaller dildo on me when he is giving me oral. One with no base or a small base works well because he still has enough room for his face/mouth. I usually need internal stimulation after a little bit of oral to keep enjoying it (otherwise I feel frustrated).
When I give my partner oral sex, I don't usually use toys. Sometimes he has put in an anal plug/toy, or I'll use a flavored lube, but for the most part, I just use my hands and mouth on him.