I thought that reds were better at smashes, blues always seem just to do ok, not great. I almost always kill with a red usmash over 80% or so unless good DI. Also, reds are the best for aerials, they have good KO potential with fair, so watch out for that (doesn't happen often but usually will be thrown in as a mixup)
Also, Oli doesn't have any grab release crap on ness, it's all just combos that he has on everyone else. The most common ones are:
dthrow>fsmash (at lower percents)
dthrow>dashed usmash (then sometimes)>uair
d/uthrow>tether depending on how you DI.
dthrow>purple toss (more likely fsmash though)
(clover, feel free to mention some more, those are just the ones i use the most lol)
Honestly, I have the hardest time performing dthrow combos when people DI away, oli has to dash after you and he cant hit you with fsmash or usmash until he gets to you (by that time you should have been able to get away or whatever)
So basically, try to DI away when he dthrows you at mid percents (at higher, if he grabs you with a purple or blue you need to DI up if he bthrows, and left/right if he uthrows), don't di towards him after the dthrow >.< this just makes him get usmash combo way easier.
Watch out for the occasional grounded tether, it will happen if you start getting predictable with your jumps/aerials. At a 45 degree angle, odds are he'll try to upB you, so be prepared with the air dodge.
With recovery, simple, just be sure to nair before you pkt (dj
before you nair of course if you have it). If you are close enough to dj on stage or to the ledge, fair them to GTFO, although their fsmash ledgeguarding can be annoying. If they space it just right then there will be pikmin raining down on you and you may need to pkt, just get away from the edge a bit first. He will probably try to edgehog you, so get pkt off in a way that you can make it back to the stage and not have to rely on the edge to save you whenever possible.
Watch out for purples O_o seriously they are
obese fat. Keep track of their pikmin line almost better than they do themselves, if they have a purple in line, keep track of it! if they have a blue next in line at high percents, don't get grabbed. Usually yellows will be thrown or used as an aerial, and at non-killing percents they may go for the grab with them. Whites, keep track of where they are and if they throw them at you get them off quickly with nair<(the all powerful move). PURPLES!!! keep track of them, they'll screw you over. Don't let yourself be surprised by an OoN (out of nowhere) purple usmash! you will die... Fat men ftl in our case, i absolutely love them

*ahem* when i'm playing olimar that is...
>DON'T GET GRABBED< seriously, this is probably the biggest thing people need to do as any character against oli.
Stages
Don't go to: Halberd, YI (obviously...), BF (imo, only pick if you really like it though...)
Semi-bad/ok/whatever: Lylat (tilts, grabs aren't affected, meh), SV (ish... more even than anything), CS (a pretty good one for oli, its meh), PS1, Everything else is whatever.
Good stuff: Brinstar, and whatever else
For all Ness' stuff on these go to the stage discussion thread
Halberd:
It has a low ceiling which can lead into really early usmash and uthrow kills, the platform makes that even earlier. Olimar can usmash through the platform and hit us, except for purples (this works on both sections of the stage). There's a lot more stuff about this stage that Oli likes, it's more of a feel though. Personally, this is my favorite starter as olimar.
Battlefield:
Same as halberd with the platform stuff, usmash gets you. Also he can juggle you a bunch here, but we can also juggle him. Honestly, I wouldn't pick this to fight oli on, but it's not too bad.
Lylat:
I'm not really a fan of this stage as ness, but it isn't too bad. Olimar's fsmash and grab are **** on tilting stages, fsmash on high ground and grabs from low ground. Grabs will catch you when you wouldn't think they'd catch you, especially when they pivot grab. Fsmash will block you from approaching, and it's just annoying when used from high ground.
This stage can go both ways, just whether you like it or not.
Castle Siege:
2nd section is probably the best for him, it's pretty good for his camping, .
On the third section his pikmin pluck takes a LOT longer to perform, punish him hard for this, it sucks. Again with the tilting stage (ish), so watch out for that. Otherwise, this stage is just up to you whether you like it or not.
Brinstar: mmmmm, I don't feel like writing this though, too tired. All i'm gonna say is that this is bad for olimar, as he isn't very good in the air. Pop him up and keep him there.
I'll leave it to people who like this stage better than me (i love it).
PS1: meh
FD: meh
everything else: meh