Most likely (you will need to do a visual inspection) a head gasket section between a cooling passage and an oil passage failed, not a head gasket section between a cooling passage and the inside of a cylinder. See if all of the cylinders are clean inside, on top of the pistons and on top of the rings, and deep inside the spark plugs.
Base gaskets very rarely fail- they are under less pressure and cooler, but it is possible.
It is also possible that a head wall or cylinder wall cracked.
A leaky water (coolant) pump would less likely put coolant in the oil, but it is possible.
“Banking institutions have banished precious metals and substituted a more fluctuating and unsafe medium; they have withdrawn capital from useful improvements and employments to nourish idleness for the few, who prefer demoralizing pursuits to labors useful to the whole; the peace of the whole is endangered by evils more easily to be deplored than remedied." by Thomas Jefferson,1810 (edited)