Well, DanGR made about one post earlier about the matchup, but he didn't really explain. Also, i disagree with him in some ways about how Sheik succeeds in the matchup. I'm not arguing the matchup to be the exact win splits i have against DanGR. But my goodness, i think Sheik has a better shot than 20 - 80.
wow, I missed the whole discussion. I'll clear up some things. I think it's 60:40(maybe 50:50) Sheik for only two reasons. Sheik's aerials DO cut through pikmin, and she has one approach that's VERY hard to counter.
Olimar outcamps Sheik pretty strongly. Through lots of experience, I've found that charging needles is a waste of time. The needles almost invariably end up getting thrown into a pikmin, spending what took you a long time to charge to destroy one pikmin, which takes Olimar a milisecond to replace. If i do find time for needles, (the only legit time slot i can think of is when Olimar is falling from above, during spawn invincibility, or some other rare time when you shouldn't be juggling, approaching, or edgeguarding), i make sure not to waste them, and try to throw them when i am positive they'll hit.
A lot of Sheik's approach game (i'm talking about approaching a grounded opponent) is counted out almost completely because it's not diagonal. Olimar is really really strong on the horizontal. His grab has enormous range, and unlike most grapple grabs, not much after lag. Because it's a grab, there isn't many ways you can punish it. A running spot dodge would only work with insane timing and spacing, the spotdodge would put you out of action long enough for him to recover from the grab. Shielding is obviously not an option, vanish isn't because we aren't discussing a close range move, and you can't kill the pikmin he sends out to grab you, grabbing Pikmin are invincible. Yes... it's a clash between the two characters with the best access to invincibility frames, Sheik and pikmin.... i kid.
Anyway, the only correct response to a grab, besides a perfect theoretical spotdodge, is to shorthop. And that puts you on the diagonal, where you should be for most of the matchup.
I've been winning this matchup off of one approach, if a good counter can be found to it and there's nothing i can throw in the mix to help against the counter, then this matchup probably is 20-80, probably worse. I use autocancelling nair->anything. Usually it's autocancelled nair->jab.
very true, this one approach doesn't fail if he does it right. If I can find a way to beat this one freakin approach, I'd win much more than I do.
The reason the approach works so well is that Olimar's is either slow or works differently. If Olimar shields the nair, and tries to grab, Sheik's grab comes out first. I might be tooting an antique horn, maybe Sheik does this to some other characters too, but it's lovely against Olimar. Someone tell me about any responses to nair->jab that Olimar has, DanGR has definitely searched.
This is the reason it works so well...
If you space the nair or fair well enough this approach doesn't fail.
-For the nair-if the end of your foot barely hits Sheik.
-For the fair, you don't even have to hit olimar for it to work.
They both cut through fsmash and upsmash anyways, so you don't have to necessarily worry about an attack from Olimar, but rather the shieldgrab.
Sheik's grab does come out faster than Olimar's. If you're both on the ground, and you're as far from olimar as you can be for you to grab him, you'll grab first.
Sheik's jabs are faster than Olimar's grab. Believe it or not, Olimar's grab has startup lag. It takes a small amount of time for the little pikmin to run up and grab your leg.
Worst case scenario is that Olimar shields and just lets you push him back. There's different things you can do, but for the most part you stay in control. My favorite response used to be pushing him to max jab range and ftilting. My new favorite is much more cruel: i repeat the approach, shorthopping another nair and jabbing.
it is cruel, and there's nothing I can do about it if you do it right...
So that's the grounded approach game. Even if someone discovers an answer for that approach, i feel pretty confident Sheik can step it up and just mix the approach with something else too.
If I wanted to, I could roll around him, but she'll just not do the jabs and/ or jump again. Sometimes, when I try this, I can roll around her, and catch her trying to jab me. I can then fsmash or somethin. If I roll back, she'll jump and do it all over again.
If your opponent forgets he is Olimar, or decides after seeing the nair approach that he'd rather not be Olimar, and starts approaching you, it's probably better to stay grounded. You can shield grab some of what he'll do, and vanish a lot of what he'll do. I love vanish to death, I can't go very long without saying that.
this is what I tend to end up doing. I approach her so she won't approach me. 
In air versus air combat, it's harder for me to describe. I often use fair to hit Olimar slightly before his fair or bair comes out. Nair can do it to, I don't know why fair keeps popping up in my matches. If the hitbox of your fair, and not much else of your body is hitting an aerial pikmin (or an upsmash pikmin), you tend to hit the pikmin and hurt it instead of taking damage. That's what DanGR talks about sometimes when he complains about low priority aerials, but i don't think it's actually priority.
It is definitely priority. I invite anyone to test this out. I can fair, bair or dair before she nairs, bairs, or fairs and it'll knock my pikmin out of the way and hit olimar. It's annoying. Sheik's nair, fair, upair, bair, fsmash, upsmash, and ftilt all have priority over all of Olimar's pikmin attacks; fair, bair, dair, upsmash, fsmash, and dsmash. Test it out. I'm not even joking. It's not necessarily her attacks are quicker. It's that they just cut through pikmin. that simple. It's just like Kirby-his aerials just go through the pikmin. There's a very select few characters that have this same exact trait that's bad for Olimar-MK, Marth, Kirby, Rob, Wolf and Yoshi. MK, Marth, Kirby, and Rob are Olimar's only disadvantages, IMO.
If Olimar is below you, and you are both in the air, he pretty much has the win on you, you need to try to air dodge and find land like a transatlantic sailor. If you are below him, both in the air, and you have no stage below you, again you need to try to just survive. (usually you aren't loaded down with options in that situation.) If you are below him and he's up in the air, for the most part you can beat him, hitting him before his dair comes out, or waiting for him to use it to early.
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