You can't paint over the Oil stains. Wait, I mean you can, but the paint will peal off the oil stains.

You need to get a proper cleaner to clean the oilstains up, then, if the floor is smooth, you'll need to etch the floor with a muratic acid. Once that is done, make sure it is clean, then use a proper sealer/primer first, then your paint.

I just did mine, but it was brand new, and a rough finish so I did not need to etch. I used a primer sealer that is a bit tacky. I followed the instructions exactly and did two coats of one-part epoxy and got outstanding results. However, I just did it, so I'll let you know in 5 years how it holds up.