Well my friends a nasty Olimar main so i think i can help =)
First of all, as Marth, YOU have the advantage of spacing with your sword, while Olimar has reach with his Pikmin. For the most part from what i've been through, your attacks and smashes SHOULD have priority, so use that to your advantage, though his grab has distance with blue/red/yellow pikmin, so watch out for those. I usually try to keep Olimar on the ground since i don't want to get Fair'd because it does alot of damage, and it's also because Olimar's moves are too slow to keep up with Marth's on the ground, so use some chain combos and smashes to increase his %. He may still short hop+Fair, but who doesn't do that. =/ Try and stay on Olimar so he doesn't pile up damage on you, because he can, and FAST. YOU have to be faster.
If a white pikmin gets thrown on you, go on the aggressive on Olimar so you take the Pikmin off while piling up damage on Olimar. You don't want to stop and kill the pikmin. Olimar WILL take advantage of that if you leave yourself open.
The best advantage i feel in the matchup is edgeguarding. Olimar's biggest weakness is his recovery, which sucks and can be countered pretty easily. All you have to do is either Fair, Dair to a spike, or just hang on the ledge and bam, your up a life.
Hope i helped...