Stick your finger in it and determine if it is in fact goey like oil or if it is a heavy dose of fuel residue smell helps alot. If it is oil then you have a good size issue as it means you have enough in there to fill the exhaust enough to blow back. The next step would be to check the plugs and determine where the problem is. If it is just fuel then you settings are off somewhere, carbbed, check floats, TBI, check regulator, MPFI, check MAF.
There should be some decisive tell tale signs of these conditions though. Oil can be because of a couple of reasons but the plugs will let you know whether it is an internal or external cause. You may also want to check the intake, hose, TB, etc. Then just post your findings with a little more info on running conditions lately, and specs. Someone will help you diagnose once the info is here.