This was seen as one of Diddy's most difficult MU's since the beginning of the game, and it stills holds true. If you want to iron out the MU though you need to really download the opposing Olimar and try find a way to infiltrate his defenses. This is true for any character but since Olimar is so frustrating to deal with, you'll need mostly reads to break through his mid-range neutral game. I would say you should direct your punishes to throwing him off-stage since Diddy's ledge trapping is the best in the game, and can easily smack Olimar with any aerial off-stage. Most importantly though, you want to keep on the pressure. Don't let the opposing Olimar get at distance from you, direct your footsies game to fighting him up close and keep him there. Hope this helps.
