What if i disagree with the Sacred Seven? I think Olimar should be just above ROB, and that Diddy should be just above Dedede.
Olimar's matchup spread is really really good. He's a counterpick to Snake, Snakes will concede it. Some say he's a counter to G&W, some says he's not, either way, he plays a tough game against the second and third best characters in the game. He does more or less neutral against the rest of the characters in high. Except he counters Dedede very very well.
He's also very similar to G&W in that when he plays mid and low tiers, he wins by landslides. Olimar and Game & Watch both have many matchups where the advantage is so huge that they don't even have to think.
One reason why is that a HUGE proportion of the characters in this game like to hit shields with retreating aerials. Olimar can grab tons of those (it's usually a bair). Off the top of my head, i think he grabs Wolf's, G&W's, DK's, Sheik's, probably any of luigi's, Kirby's, Dedede's, CF's knee.. it's probably faster to name who can manage to go in the air, hit his shield, land, and not get grabbed. Probably just Wario, Jiggs, Yoshi, Ness maybe, and a few i can't think of. Anyway, many of these characters have to change strategies completely, while fending off the best spam in the game (arguably in competition with Tink's), and trying to get in and hit one of the best defensive characters in the game.
Olimar's most popularly called-upon weakness is eroding quickly, that is, bad recovery. Olimar has never had bad horizontal recovery, because he's floaty, his problem was getting the vertical he needed. I think Olimar's recovery problems were exaggerated early in the game, when the usual response to an fsmash is DI in the opposite horizontal direction from the move (left for a right fsmash). I would love if that was still the most popular response, but now everyone DIs upwards. He pretty much uses the force of the hit to get him high enough in the air that his vertical recovery issue doesn't kick in.
Even if he does get stuck needing to use his tether recovery, half the time he has a purple (if it's not next in the lineup, he can empty bair or throw to get to it). Purple's knock off edgehoggers. If you get off the ledge when you see it coming, he'll grab the edge.
And if he doesn't have a purple, he'll have a white. You'll kill him allright, but you have to stay on that ledge for a pretty long time to hog him. He'll throw a white on your back, usually with a red or yellow to keep it company, and you have to take double digit damage in exchange for the edgehog.
His longevity is also improved by more advanced usages of the whistle. If he gets to 140% or so, he just starts whistling your fsmashes and other hits. It gives instant super armor. It doesn't make him invincible, but he basically gets enhanced spotdodging/shielding/airdodging abilities when he's at such a high percent he knows you can kill him with anything.
As you should all know already too, he has a strong air game, forced downthrow combos, a fast uptilt that outprioritizes Wolf's fsmash and most any other non-disjoint move. And he has one of those fast, strong, beast downsmashes.
Diddy: I dunno i just think he's a good character. I can't really back this one up so i'll let it go.