I kinda approach this matchup with the intention of timing olimar out. He's pretty bad at doing any damage to you if you're on the other side of the stage unless he throws Pikmin (which nair destroys). If you can get the lead (stock lead is amazing, but 50+% works too), then the match is yours. Just don't approach, floatstall extremely high if you want to, nair/bair away his pikmin, throw turnips constantly, and never approach. Oli's uair is extremely easy to see and avoid. His up+B is avoidable as long as you keep away from the 45 degree angle.
If he doesn't approach you, then you win via timeout as long as you don't take too much damage. If you stay above him, then avoiding his attacks is pretty easy, and turnips, punishing his landings, hitting him in the air, and shield pressure can slowly rack up damage. Timing Olimar out is always a viable option, especially considering how early his usmash kills... Sometimes it's best just not to get close. If you do approach him, then he will most likely just wreck you with OoS usmash, pivot grabs, aerials, etc, so its not advised...
Ummmm.... I've never been too good at it, but if you wanna avoid his constant OoS stuff against your attempt to kill kill him, then you can attempt the throw/hit him off stage->turnip/aerial->edge hog...
If the Olimar likes to air dodge right before they land on the ground, then float canceled bair is amazing. Launching olimar into the air is a great idea if you do want to play aggressive, because he can't do much beyond his amazing ground game.
Yeah.... time out Olimar. Keep him in the air. Don't approach.... ever.... Edge guard. Success
With lead, 60-40. Without lead, 45-55.