If you really have engine oil in the coolant (and no possibility of mistaking it for transmission fluid -- I assume you have a manual transmission), a blown head gasket is the only remaining possibility, isn't it? [think of the famous SH line, admittedly crudely paraphrasing here, "after eliminating all other possibilities, whatever remains, however improbable, must be the truth"].

The head gasket is the weakest point that separates oil and coolant passages -- yes, a cracked block could also be a possibility, but wouldn't you prefer a mere blown head gasket to the thought of a cracked block?

Which way the oil, or water, flows depends on their relative pressures, but also the nature of the head gasket defect.
Oil pressure, when the engine is revving, can far exceed the maximum (hot) coolant pressures (~1 bar, or ~15psi) by 3:1 (i.e., 45 to 60 psi) or more, so oil going into the coolant is likely; but at idle, coolant pressure can exceed oil pressure (~10 psi); so both directions are possible.
Unlikely but also possible, certain tears in the head gasket's material might act like a valve, preferably allowing flow to be easier in one direction than another; you could have oil in the coolant as well as coolant in the oil, or both.