I feel like a fool for asking this question because I am mechanically inclined and I know a reasonable amount about how engines work but somehow I cannot comprehend how this process manages to work at all.
Take your typical car engine. Oil lubricates the moving parts and gas (or diesel) gets burned as fuel. However, in my mind at least, oil is needed in the piston cylinders to keep them lubricated while they move up and down as the gasoline (or diesel fuel) ignites. Why is it only the gas or diesel fuel that gets burned off and not the lubricating oil along with it? Motor oil will burn easily at high temperatures yet having that happen in any significant amount is a very bad thing.
The piston rings keep the oil from entering the combustion chamber. Only an extremely thin film of oil is left on the walls of the cylinder, and the temperature of the cylinder wall is not hot enough to ignite that oil (at least, not much of it).
As beowulff stated, the piston rings, especially the oil control ring, keeps almost all of the oil from getting into the combustion chamber from the crankcase.
There’s another area where oil can get into the combustion chamber or the exhaust system: the valves. To prevent that from happening there are seals on top of the valve guides.
I have had more issues with valve seal than I have oil rings. Valve guide seal s will display smoke after idling at a stop sign or just starting up. Oil rings will smoke more under acceleration.
There’s a great deal of thought and engineering to keep the oil on the outside, where fast moving parts meet stationary ones and thus, need lubrication.
There’s even more effort put into keeping the air and gas burning efficiently, but separate from where the Motor Oil is.
As has been mentioned the oil control rings keep oil out of the combustion chamber.
A piston typically has 3 piston rings an upper compression ring, lower compression ring and an oil scraper ring.
One of the by products the quest for better gas mileage is car makers are using rings with less tension to lower the friction in the engine. This can in some cases cause serious oil consumption.
A classic “amateur mechanic” story I once read about (or cautionary tale?) was the guy who installed the liston rings upside down. They are designed to scrape the oil down the sides of the cylinder. The wrong way around and you scrape the oil up, burn it like crazy, and eventually flood the engine - or so I’ve been told. I know less about mechanics.