Edguarding will win you this matchup if you are able to get him in this position_ Having come from previously maining Olimar i feel that this matchup is in Oli's favour by around 10 so 50 - 40 Oli_
Whistle gets him back on the stage effectively, he can outcamp Shiek and bait pivot grabs_ He plays a fairly strong ground game and most Oli's won't be fooled into playing air games if their opponent has a better air game then them_ With whistle + PS i find Oli to hold his own on the ground_
BF is Oli's best neutral imo and i used to love getting taken there_ Platforms give him alot to work with if you let him under you_ On the flip side, platforms can work fairly well for Shiek if you manage to juggle him and keep him above you_
If you're up against an average Oli then FO is a good choice for a CP stage_ Be wary, as mentioned before, that most Olimars will be familiar with this CP because of getting taken there alot_ Rainbow ride is another viable option if they don't ban it as they are being forced to stay airborne alot_ Keep in mind that whistle will most likely be abused alot on this stage_ Port Town Aero drive is a good option if it is allowed in your state as the lack of grabbable ledges and constant motion will keep Olimar in a fairly vulnerable state_ You can pressure him harder, keep him airborne and stop him plucking pikmin alot better on this stage imo_
I would say to ban Luigi's Mansion or Halberd_ Suss out your Olimar player first before deciding which stage to ban as some have there preferences_ LM lets him camp hard and forces the approach, high ceilings are great for vertical kill characters, alongside the mansion cielings, Oli's bthrow will make easy work on most people_ It also has good recovery options and sharking_ Halberd of course is great for it's low ceiling so make your ban depending on what type of kill character you are_ Your best option for CP's are stages which you know are going to keep him moving around alot and allow you to get under him via platforms_
My biggest issue with this matchup is the fact he can outcamp and most good Oli will be able to bait the throw and keep grounded against you_ You're moves come out faster i'm fairly sure so once you're inside his wall then you need to keep him juggled, airborne or edgeguarded as much as possible_ jab > grab is a good option if you can manage the approach_
Feel free to correct me if i'm wrong with anything as i haven't had the chance to vs any Olimar mains since switching to Shiek as i was the only Olimar main in my state (and kinda made people hate him alot)