Joergen,
You have oil in the crankcase and at the lower portion of the pÃston and cylinder anyway (along the skirt, up to the scraper ring, that's why it is fitted to the piston). When the valve guides are worn otoh, oil will get into the combustion chamber from the rockerbox and will be burnt there, resulting in blue smoke from the exhaust, the same will happen with worn rings, with oil going past the skirt up to the combustion chamber.
The other effect is lack of compression and excessive so-called blow-by, meaning that a large amount of the combustion pressure will escape past the rings into the crankcase, building up pressure there and causing excessive oil leaks from the breather.
So yes, worn rings COULD be the problem. But if the engine doesn't smoke and lack power, I don't think this is likely to be the case.
Anyway, a compression test will tell you.
Cheers, Markus