Where is the oil drain plug on the 1986 883 sportster?

It will depend upon whether you mean the engine oil drain or the Primary/Transmission oil drain. The oil drain for the engine oil is actually NOT a plug. Underneath the engine is a hose that is clamped to a boss on the frame. Loosening the worm clamp and pulling the hose off the boss will cause the oil to dump (you did put the drain pan under there, didn't you?)