May be that oil got past the rings in the cylinder. This could’ve occurred when it was tipped over, or you may have overfilled it when you added oil.
First be sure the mower is not overfilled with oil. If it is, turn the mower up on the oil fill tube side, with a pan in place to catch the oil, remove the oil fill cap & allow the oil to drain out into the pan. After the oil stops coming out set the mower back on all four wheels. (I wouldn’t advise trying to use this oil in the mower again, by the way.)
Next, disconnect the spark plug wire from the spark plug, remove the spark plug & inspect it for evidence of oil in the cylinder (and to see if it otherwise appears to be in good condition). While the plug is out, (and without pressing the fuel primer bulb, etc.) pull the start cord several times (you might see oil being discharged out the spark plug hole). Clean the spark plug (or replace it) & reinstall it, then reconnect the plug wire securely.
Add new engine oil back into the mower, in increments, being careful not to overfill. Screw the fill cap back in place.
Ensure it’s got fresh gas, press the primer bulb a few times & give it a pull. If this solves the “won’t start” problem, expect to see a lot of smoke as it runs. It has to burn off the residual oil, so just allow it to run a few minutes, and the smoke should clear itself.
Good Luck.