If you're saving it as a .STL file to export to slicing software, in the SaveAs menu click the options button and make sure the resolution is turned up to where you want it.
I set my angle tolerance to 1 degree if I need really circular circles, otherwise it's at 5. Also, if your circles are oval you have a problem with the X or Y axis of the machine.
If you're saving it as a .STL file to export to slicing software, in the SaveAs menu click the options button and make sure the resolution is turned up to where you want it.