Against Olimar, the main problem is approaching him, once you're inside your able to punish as you usually do.
However, Olimar has what I consider the strongest ground game in the entire game, approaching him isn't easy and if you plan on getting any damage at all you HAVE to approach.
Nair might do the job... unless the Olimar pivot grabs, so I generally tend not to use it, instead I'll stay in the air or on platforms, quickly making my way towards Oli. Once you're near him, you have a few options. You could use Bair and double jump out of the way of the grab that is sure to come if you miss/he shields it, you could use Dair and Nair off of his shield or make your way behind him, unless it hits and then you get to juggle. Alternatively, you can cape stall when you think he's going to grab, and then you're free to fall in with a nair and grab or something.
Basically, get in and punish heavily with throws/your ground game, and try to get that gimp as early as possible. Of course your aerial game is good too, if he happens to be in the air, but I think the gimp is way more important.