"LMAOOOOOOOOO" is a version of the famous LMAO with 8 extra Os. When something is so funny a version of LMAO with 2 - 6 extra Os are not enough. LMAOOOO is pretty common, but LMAOOOOOOOOO strengthens the laughter even more so. If you laughed so hard you can use LMAOOOOOOOOO. If that doesn't show how hard you laughed you can also say a version of LMFAO, a more emphatic version of LMAO, with 18 Os "LMFAOOOOOOOOOOOOOOOOOO".
Head ass LMAOOOOOOOOO.