Where is the oil plug on a caftsman lawnmower?

Where gravity can get the oil out; usually under the deck, covered in grass clippings and rust. Probably a square bolt head. Either break out the 12 point socket set, a big channel lock, or suction the old oil out through the dipstick hole with a Mityvac.

Some mowers have oil filters, some don't. Change it if you got it. Also a good time to change, or at least clean, the sparkplug. 2$ part that can make a world of difference