this matchup is a lot about understanding how olimar's line works. first you need a fundamental understanding of pikmin
yellow: broken disjoint, broken uair, broken priority, bad killpower and bad throws
red: *** for everything except fair
blue: great pikmin in general. usually used for throws
white: great for latch
purple: great for zoning disruption from long range, fatass hitbox, great for smashes
LONG RANGE: Watch his line.
If he throws a white immediately get rid of it.
If he throws a purple run away or block or dodge it, generally i like to block or jump over it. Just don't get hit by the purple - even letting him pull more is better than taking the pikmin because either way he'll pull more, just if you get hit by the pik you take an extra 8% and you're in knockdown.
If he throws a blue don't be TOO worried about getting it off but if you want to get it off, kill it rather than blocking. Why? Well...
If he throws a red you should actually just try to take the damage. It'll go back to him sooner or later and reds are ***. If you are really opposed to this then block please.
If he throws a yellow same thing as blue.
The gameplan for long range changes @ different percentages
Low%: Let him latch you to ~40%. Throw momentum by Olimar is gay and annoying
Mid%: Avoid getting latched by anything that isn't a red. This is where Olimar really wants to frustrate you.
High%: If the Olimar isn't stupid he won't latch anything important on to you, so use this opportunity to unstale your kill moves on his bad Pikmin. Once he notices this he will have to take risks with latching, and you can then assert your advantage more.
MID RANGE
So Olimar actually gives Marth some trouble here.
If Marth approaches and Olimar predicts, reverse stutter fsmash AAs Marth. Alternatively, pivot grab works too.
If Marth approaches and Olimar reacts NEAR THE BEGINNING OR IN THE MIDDLE, usmash OOS is plausible. The only way Olimar can beat Marth's approach at the end with a usmash OOS is if he gets a random PS.
If Marth doesn't approach then Olimar can just change to long range spacing fairly quickly by doing b-reverse Pik Toss.
Once Olimar gets one hit off what's frustrating is that he gets guaranteed latches or gets a knockdown read.
If Marth hits Olimar's block and mashes jab while the Olimar tries to punish his attack on shield w a grab, he buffers a grab break and then he's in there. The way Olimar beats this is usmash. Marth can spotdodge as opposed to jabbing but exposes himself to the risk of getting grabbed/other punishes (fair, yellow fsmash, etc.) If at any point Olimar hesitates with his block up then you get to grab (or dtilt/DB). This is what you are aiming for.
As a side note, outside of the approaching part of midrange Marth definitely wins, His DB, grab and dtilt are all broken and fair is also trouble for Olimar.
Back to the main point: this part is mostly just about tricking the Olimar with your jumps and timing in order to bait usmash/grab.
The important part here is NOT TO GET GREEDY!!!
Make sound approaches and reads based on his lineup. If he has a yellow he is more likely to go for usmash.
He will always try not to end up in this situation with a white or a red and will use pikmin toss beforehand (which means you get a free grab or DB/dtilt) - if he happens to end up in this position, then with a white he is likely to try to time a grab at lower percents and a usmash at higher percents; with a red he is more likely to try to usmash you at lower percents but can do either option at higher percents.
If he has a blue then he can do anything.
Purple he's likely to usmash you but if you're obvious + he has good reaction he'll just grab.
If at any point you feel uncomfortable vs Olimar in the mid range just DON'T COMMIT IN A WAY THAT WILL LEAD TO THE WORSE OUTCOME (between grab and usmash). Olimar is looking for that commitment to punish you. You should be OK with taking red/yellow grabs for commitment/white usmash, etc.
Like Kadaj said, once you get Olimar's shield with a few moves you can actually straight up DB and it will chip his block. After you have him expecting DB you can play cool tricks with grab or partial DBs if you're feeling really brave.
IN THE SHORT RANGE:
Don't box Olimar, lol
Marth's grab and DB work wonders here. Dtilt can't be punished by Olimar unless he PSes iirc...haven't tested this in a while.
Most of the time Olimar is looking to panic usmash here because he doesn't have anything else. If he isn't usmashing he's spotdodging or rolling to try to escape pressure. ABUSE THESE THINGS!!!!!
Not much else here. If your Marth is competent then once you have trapped Olimar here Olimar should always be put into a bad position afterwards with grab or DB, which leads us to...
ON JUGGLING:
Just watch Mikeneko.
Well, to be more specific: FF everything. FF uair is your friend as it covers a huge amount of horizontal/vertical range.
Once you've conditioned him to expect you to try to uair he will start whistling.
Fair him for it or grab him for it dependent on where he is. Sometimes you will get a cool bair off.
Overall Olimar is *** when it comes to being juggled. Don't be fooled by his b-reversal whistles! If you FF Uair you will be safe no matter what (provided you're doing it properly of course). This is where you play like a robot.
ON THE LEDGE TRAP:
Standard ledge trap works wonders here. The roll range is about 3 or 4 Olimars away from the ledge. His ledge attack is really bad. Pay attention to whether or not Olimar has a purple or not, as it's the only reliable option he has of getting you away from the ledge. If Olimar does that little uair thing just dtilt the pikmin if you can but don't get hit by it.
If Olimar throws something at you, just crouch and hit the Pikmin. This is a good sign that the Olimar is getting antsy. Remember that Olimar has a limited number of latches to the ledge - it's either 3 or 4.
If Olimar does a ledge jump, DO NOT JUMP AFTER HIM UNLESS YOU HAVE A READ. Stay grounded. FF Uair is your friend. You need to be landing before him if this scenario happens. If the platform is moving away from you like on Smashville then it is 100% better to let the situation reset rather than chase unless you have a read. Be safe.
If Olimar does a standard getup, then depending on his line stand at the optimal range (if you didn't read the getup, that is). He doesn't actually have any options but to jump (yay Olimar is above you), spotdodge (wait some more), roll (LOL), block (dash grab or dtilt), and wait (wait some more). Olimar sucks on the ledge!
ON BEING JUGGLED:
Landing with nair is problematic but beats usmash (sometimes). You don't actually beat grab this way usually unless Olimar does it really early. Don't nair.
Watch for yellow uair!
I don't have too much trouble with being juggled but what Olimar is looking to trick you into doing is a bad fair or nair so he can get another usmash or grab. Being juggled is mostly about reactionary play and just taking certain latches/taking certain punishments in favor of being on the ground. Don't worry about hitting Olimar on landing, just focus on reacting to his movement/actions in order to get your two feet on the floor.
ON BEING LEDGE TRAPPED:
Dude this sucks. Olimar's fsmash is really gay and NEO-style planking is not 100% effective. RCO lag only exacerabtes the problem.
What you want to do is just try to react to Olimar's fsmash and the pikmin pull and strike where there's a difficulty in timing. That's the aggressive method. The less aggressive method is to just do a ledge jump and escape but this isn't exactly the best strategy on every stage + if Olimar reads it he gets a free fair and then you're ****ed.
Like every other matchup when you're on the ledge, just be patient. Olimar is very short so Nair isn't really the best unless you are aiming to double hit and prevent shieldgrabbing. Watch his line, as usual, and accept certain punishments in favor of getting back onstage.
ON STAGES:
Assuming Unity, banning PS1 or CS is a good idea. It's all about preference. I actually like FD a lot vs Olimar as you get to run away vs pikmin and work your juggle/ledge traps really well. It's a double edged sword though, as you don't get platforms to help you be tricky on the approach. BF is the best neutral vs Olimar, and SV and FD are tied.
Yoshi's and Lylat are better/worse for Olimar depending on the style that the Olimar plays. As for Marth these stages offer different advantages/disadvantages for him. Lylat allows you to pressure Olimar harder when it comes to making him choose between stage and damage but the tilting messes with your spacing hardcore (plus he gets more yellows). On Yoshi's the stage is smaller and the height of the platform allows for some cool **** with Marth but the slants are not that great and Olimar's traps are strengthened on this stage.
Delfino/Halberd are fairly unique but I don't get to play on them often as Marth vs Olimar for some reason. I don't think the strategy should really change on either stage that much but remember that Olimar gets some great pull rates on Delfino sometimes and you just have to camp it out if he has the great lineup.
Brinstar/Cruise, lol Marth *****. Just do Marth things on those stages. Don't underestimate Olimar's tricks on Cruise tho
If there's one thing I would stress about this matchup, beyond anything else, it's watch his line. Watching his line will get you farther than anything other piece of information in this matchup, as it's where Olimar is the MOST different from other characters imo.