I also have the olga circle cut rotary cutter. I have never been happy with how it works. I have had the best success when I layered only about 3 pieces of fabric and put a piece of paper on top. I use loose leaf paper. I does dull the blade some. Not sure about denim. Some of the LQS here will let you use their acquit cutter for a small fee.
What are you making with all of the denim circles? I have a bunch of denim and haven't decided what to make with it.