Nair is your friend, and seriously the Tornado Spin is amazing against Olimar, gets you in range quickly, while negating his pikmin throws, and usually before he can get off a smash.
This right here.
My friend plays a pretty good Olimar. I've found that with the pikmin and Oli's range, you can't chase him on the ground if you're on a rather big platform stage (FD, BF, etc) especially since luigi slides so much when he's hit.
What I found that works out great is to use the Tornado to chase him, if he throws a pikmin, be ready to act quick because it will cancel your tornado (Though you won't take a hit either) when that happens BAM tornado again almost instantly. Once you catch him, if he's pretty good at shield grabbing the tornado, attempt to tornado through him and end up behind him.
Once you get close to him via the tornado, short hop nair fast fall works great to neutralize his attacks.
He's really not as bad as he seems, especially if you keep him pressured and don't let him attack to much on the ground.
The great thing is Luigi has the advantage in the recovery department, as Oli's is terrible imo. Once you get his %age up, start bair'ing him off the stage. When he needs to rely on his tether to get back on, /lol at him then hug the edge. Tetherer's can't tether if you're grabbing the edge.
Hope that helps!
P.S. Oli's utilt/usmash is pretty mean, watch for it. I would suggest airdodging into an attack when you land as opposed to nair fast fall if he's got you in a juggle.
