Two things are going to happen if you continue to drive without your oil cap in place. First you run the risk of an engine fire from the oil spraying out into the engine bay. Second you run the risk of running the car out of oil and locking up your engine. If you havent already done so catch a ride to the parts store and buy an oil cap and go clean your motor up. Do not stick a rag in the opening and continue to ride around (I'm not saying you did or will do it but just in case).