I used to suck at this MU, but now I feel it's actually one of my strongest right now, albeit it difficult against the top level Marths.
I skimmed through what everyone said, so I'm just gonna post everything I know in my typical Rich fashion:
First off I currently feel this MU is only 55-45 in Marth's favor. We have options.
As Fino noted, I swear by the stutterstepped fsmash in this MU. It's perfect for when marth is jumping in and is committed to the fair.
The smart marths will jump at you, then right before it seems like they are gonna attack they will double jump and land a fair on you. Predict this jump and Uair them. You can actually Uair Marth a lot in general, but you can't really force it. It's a prediction thing.
I prefer to Dthrow-fair marth. Most marths will immediately try to fair you back, so land, shield the fair (it will be poorly spaced) and shieldgrab it. Normally you can't shieldgrab a well spaced fair but in this situation you normally can.
Speaking of which, just try not to let the fair hit your shield at all. If it does hit your shield, wait for the jab, or the sideB or the sidestep. If he grabs you, after fair, so be it.
Avoid rolling. A lot of marths will just jump at you to see what you do, then punish your roll on reaction since Oli's roll is terrible. Utilize other options, like retreating a shorthopped nair when you wanna gtfo.
Make sure you space your fair on his shield if you decide to do that (this goes for every MU). Oli's fair cannot be shieldgrabbed ever (unless powershielded). If you ever get your fair shieldgrabbed and they didn't powershield, you should feel bad about yourself.
Unless you have hella purples, don't sideB very much except to force an approach if he's really far away.
Also don't jump for no reason, establish your ground game.
Utilize your walk. Make Marth commit to something. Don't run around trying to pivot grab and do all this unnecessary bull**** because you're probably giving up space to Marth for no reason.
If he's on the ledge, just throw out a bunch of fsmashes. Alternate the timing. Charge 2 fsmashes, then do a regular one, then a charge the 2 regulars, whatever. Be random. His best option is to drop off the ledge then jump-nair, so be ready for that. If you hit him far off the stage, feel free to tether the ledge and try to screw up his timing. I know it's risky go for stuff like that normally, but I realized recently that if we are just a little smart about it we can make Marth upB on the stage which results in a free hit, or even occasionally gimp him.
If marth misses an upB on the stage and there is no platform for him to land on, line up a double sweetspot dsmash (unless he's at usmash kill % of course). That will do like 26% minimum (side note: double sweetspot fully charged dsmash with 2 purple pikmin does 52%, wtf)
But yeah just be smart about shielding, don't wait in your shield for no reason, stutterstep that fsmash, throw in a pivot grab here and there. Be smart about dancing blade too. That move is the ultimate rock paper scissors. Just know that he can alternate the timing, stop the move and hit you with something else (or wait for you to roll) or complete the move. Just make sure you mix up what you do as well. Don't roll behind him every time of he'll catch on. Your best bet is typically to try to grab the move.
As for recovering, Just make sure you're DIing well.
**** it I'm gonna write a whole paragraph on recovering
When you're recovering, make sure you're holding up and slightly towards the stage when they are close to you. Cstick your aerials and make sure you're DIing while you do the aerial in case they try to beat it with something.
Speaking of which, for some reason most Olimars (and players in general) assume that when they are recovering they have to make a defensive decision (airdodge, whistle, etc). We forget that sometimes we can attack. This works a lot because players will try to bait our airdodge/whistle by jumping at us, and we can just fair them back and recover safely. So keep that in mind.
Also, when you're recovering towards the stage, you don't have to move in the exact same trajectory towards the stage the whole time. It's ok to sometimes fall straight down or even DI away from the stage to mix up your movement and throw them off. Imagine for a sec:
If a target is moving from left to right, it's easiest to hit if you just follow the target and shoot where it's about to be. But if that target is staggering its movement and sometimes moving left, then going right for a sec and then going left again, maybe stopping for a split second even, that's way harder to hit. Keep that in mind as you're going back to the stage (whistle bounce!!!!!)
If you whistle an attack, you can usually just upB them or hit them with a nair or something. Uair/Fair are wonderful too. Also, if the ledge is open, TAKE IT. Olimar's upB with 6 pikmin goes far, so if they go out to chase you, just upB without using your double jump. Worst case scenario is they hit you and you just recover again while saving your jump.
Holy **** this post is long
Yeah that was pretty much my mini guide to recovering as Olimar.
As for killing Marth, idk I just kinda kill him. I don't pay attention to that ****. lol
I think that covers everything :3