I have never lost to NC-Echo's Olimar (at least so far, and I've played him quite a bit), so I think I'm qualified to give a fair assessment on this matchup.
Marth:
-Aerial control
-easy, early gimps
-able to apply lots of pressure despite Olimar's range
-normal zoning (obviously will be varied cuz of grab range and Fsmash range) can hurt any Pikmin that Olimar attempts to throw (forward B)
-can juggle somewhat easily
-tips set up very well for edgeguards/kills
Olimar:
-awesome grab range
-powerful, far reaching, and quick smashes
-whistle can disrupt juggles, along with AD
-can use tether recovery to gimp
-can legitimately combo at low percents (usually from a grab)
-great AD following with Usmash, Up-B, and grabs
This is much easier for Marth than Olimar. Marth has to play a slightly varied zoning game, while Olimar must camp and attempt to grab Marth as he zones onstage.
Once Olimar does get a hit however, things do get fairly bad for Marth. Early on, grabs can combo into Usmashes, Fairs, Uairs, etc. Depends on the player really. Easily in the neighborhood of 30% if done correctly from a Dthrow early in the stock. Olimar must maintain the control that he gets from these early combos because Marth will eventually pay this back with a gimp or multiple Dancing Blade punishers if Olimar isn't wary.
Marth obviously doesn't want to get grabbed at low percents, so the game plan usually consists of more cautious zoning earlier in the stock, followed by more of an aggro type playstyle when Olimar gets into mid percents to force him off of the edge (not difficult due to weight). Marth is excellent at punishing Olimar with empty SHs and Dancing Blades, which will eventually push Olimar to the edge. Olimar has even less options on the edge than when he's fighting Marth at the center of the stage. Marth can work in some grabs here, or more DBs or even a Fair (never a Nair!) to push Olimar over the edge.
At this point, Marth has it made. Olimar will most likely DJ to gain some height so that he can whistle/AD through Marth's attacks as he falls. A Fair seals his fate there. When/if Olimar adapts to this, then his only option is to delay the DJ, which means that Marth can fake doing a Fair (run off stage to FF to DJ back) to force an AD/DJ AD, and will punish accordingly.
Olimar can do decently against Marth offstage as well. Olimar is, of course making a risky move if he decides to jump out at a recovering Marth (it won't be very far off the stage, but close enough to make a difference), but should Marth lose his jumps and get Faired by Olimar, then he can be edgehogged very easily due to Olimar's tether regrab to renew invincibility trick (Marth's best counter to this is waiting for Olimar to tether and Up-bing at that point to hit Olimar off and reverse the situation).
Both can kill pretty well, Olimar can kill around 100% and above with some throws, and his Usmash, Fsmash, and Dsmash all come out extremely quickly and can kill (105%-120% I'm guesing, don't exactly remember the numbers), or at least set up for a fairly easy edgeguard at moderate percents (60-90%).
On the whole, Marth should have control of the match most of the time because Olimar can't disrupt his zoning when he's sitting in shield and hoping for a grab/is too far away to Fair OOS.
I'll say 65:35 for now. Thoughts?