Oil into the combustion chamber would be:
- valve stem seals
- sucking oil in through intake
- bad rings/broken piston
Theoretically bad head gasket, but you would see water sooner than oil (water is pressurized, oil is gravity feed from valve cover area), burning water/coolant would be white smoke, oil is blue.
Spark plug end bouncing around causing chip/crack in combustion chamber dome would not cause oil to leak in.
Oil coming in through intake would most likely be sucked in from PCV or from oil filler neck into throttle body (this would be excessive blow by of combustion pressure into oil pan up into lifter valley into valve cover area out filler neck vent tube).