Toronto doesn't have very many rooftop bars. Oasis is one of them where you don't have to pay cover (I've been on week nights and weekends and have never had to) and the vibe is very relaxed. Drinks are reasonably priced and the DJ is good. There are tables to sit at to chat with your friends, and you can also order food off the menu.
The only downside is that it's surrounded by condos, but other than that, it's a great place to spend a Saturday night outdoors with a beverage in hand!