Almost everyone in my crew plays Olimar :x I'll edit in my detailed info in a bit, but the main things to abuse are the fact that Pikmin can be killed mid-attack (even Smashes) or mid-Grab if you time attacks right, and even if you are about to get Grabbed you can attack Olimar (or throw a Banana) before his Pikmin reaches you to stop the Grab. Olimar has NO Grab Armor, which you need to abuse because that's half of his momentum-stopping game. I'll admit, doing all of the above is HARD... but possible.
Peanuts stop his thrown Pikmin, as already stated, and more importantly his Pikmin stop your Bananas, making getting to him very difficult. Once you do, however, if you can blow him away from his Pikmin, unless they Whistle in mid-air they won't be able to attack you with normal Pikmin moves.
Try to force Olimar above you through Banana placement and pressuring; it's one if his weaker situations.
You need to stay out of not just Grab range, but PIVOT GRAB range; it's HUGE. This is especially important while falling, because Olimar can abuse the heck out of Diddy's poor horizontal aerial speed.
Learn the trajectory of Olimar's F-Smashed Pikmin as they go off the stage; this is a common edgeguard you'll see and it will nerf your barrels if you can't avoid them.
Attack when he plucks, but be very careful; a good Olimar can cancel his pluck into an immediate F-Smash or U-Smash, so you need to stay out of range of those moves even if he's getting new Pikmin.
F-Air is an incredibly important move in this match-up. It stays out for a long time, will kill Pikmin he is using to attack, and has great range in the air (and is hard to intercept on the ground if you're approaching him diagonally). This is a great way to punish Short-Hopped Pikmin throws or other times Olimar decides to go airborne.
Dash Attack and N-Air kill Pikmin, but the former is telegraphed and, on small stages, will likely put you into range of his Grab. N-Air is just plain laggy unless you use it close to the ground to cancel it.
If Olimar grabs a Banana, expect him to camp you with Pikmin throws to mix-up when he throws the Banana; do NOT fall for this! Just rush him with F-Air or Dash Attack and take the few Pikmin latches, for he can't use any of his really long-range moves like this. Attack quickly, such that if he throws the Banana you'll catch it with your attack.
More stuff later if I can remember why I lose so much vs. DanGR, other than basic skill difference. As always, please correct me if I'm wrong with any of this.