I made the same mistake once and had quite a mess to clean up.
When completely empty my walk behind mower holds 18 oz of oil.
Toro manual also says that fuel tank must also be empty before tipping over to change oil, etc. If emergency, then fuel must be pumped out. This is why I change the oil first thing in spring before putting in fuel. Of course, I won't get the non-warm oil out completely.
I had a customer that brought me a push mower he thought was seized since it wouldn't pull over. I noticed oil dripping from the muffler. Turns out he had stored it on a tool hanger or something in his garage with the spark plug part of the engine facing down. All of the oil ran into the cylinder.
I pulled the spark plug, cranked it over so all the oil came out, poured some in the crankcase and it fired right up. I charged him just for the oil I put in the mower and advised him on what happened.