I had an internship with the professor I'm applying for. Is it possible to ask for a recommendation letter? I talked to a professor in a top US university and he asked me to apply but he said finally admission committee decides. If I go to his lab for a summer internship, is it OK to ask for a recommendation letter?

The point is that I'm applying for his position so I thought maybe his own recommendation letter works well!
Yes! Definitely! I worked for my eventual PhD supervisor as an undergrad for a semester before I applied to grad school. He wrote me a recommendation letter, and I got in.