Check your oil with the engine hot, and wait about five minutes for the oil to drain into the pan. Checking it cold will give an inaccurate reading.
You're lucky with that filler cap. When I was in high school a friend of mine had a Chevy Nova. We filled his crankcase after an oil change in shop class. He forgot to install the filler cap. When backing out, the filler cap fell into the fan, and the fan shot the cap out through the rad like a missle. Since that day, I am a fanatic about checking the filler cap.
Oil in the cylinder will cause the smoke. The new plugs may be fouled, and the only way to tell may be to pull them. How is the car running now? Any more smoke? Did you get a CE light?
Before pulling the new plugs out, drive it for a few weeks at leasts and see what happens.
1998 C230 "Black Betty"