Having been an olimar main all of brawl's lifespan, I can offer some loose insight into Olimar's weaknesses. Obviously, he plays a lot differently in this game, but there are some things that carry over.
First, and this may seem pretty obvious, try to approach him at a sort of 45-degree angle. That is, use a lot of sh'd aerials that aren't easily beaten out by his fsmash or grab. The normal linear dash attack/dash grab mixup isn't gonna fly as well with Olimar, as it seems a lot of people have already figured out. Olimar, especially now that his albeit situational Brawl UpB is gone, has a hard time defending that sort of cone projecting at a 45-degree angle from his helmet. Now that his usmash is a lot slower and clunkier (though still good), I think the diagonal approach is even better now. Of course, if he's getting used to your sh'd aerials and just pressing shield, you can start doing some empty short-hops into grab and what have you.
Secondly, Olimar still has some serious issues with landing. Similar to when he's on the ground, he can't very well cover diagonal approaches to his body in the air. M2K I think mentions this in his MK vs Oli matchup article. Luckily for us, we main Falcon's uair, which we almost always throw out kind of diagonally-below the opponent. So once Olimar's in the air, Falcon's juggle game is even better than normal, since Oli doesn't really have many good options to trade with uair. And if he's airdodging, that's definitely preferable.
I also think that Oli has a really hard time recovering against falcon. Bair eats his recovery for breakfast, and if you're really going for style points, I find that his recovery is slow and predictable enough to land UpB's on. If he's recovering high, that's completely fine for falcon. If he's recovering underneath the stage, we're fast enough to run to the other side and continue the edge guarding.
Lastly, and this should seem pretty logical, don't kill pikmin with your smash attacks. Olimar thrives off of punishing stupid stuff like that, and best-case scenario it'll stale your smashes.