hope this helps, my personal summary on the matchup for our matchup thread, just to give you some ideas on what i personally watch out for in the matchup, and things we are able to do that you may not be aware of. a good read, regardless
1) recovery -- i suggest you learn all the possible recovery techniques, especially for this matchup, if snake gets you off it's extremely difficult to get back on if they are patient and know the matchup.
2) don't rush in or be too offensive, if they pull out a nade the nade will stop anything you can do, and that damage against you is far worse than it is against him. so, try to stay right past his ftilt range if approaching.
3) camp as much as possible, don't camp too far or he can out camp you, make sure the nades he throws can go past you enough so they won't hit you.
4) The best way to keep advantage in this matchup is to get snake off the stage and look for an edge guard, or follow him if he up's into the sky and recovers from the top. Expect the b-sticked grenade (where they change their momentum in the opposite direction), follow it and grab as they land.
5) Grabs are win. But, spamming them is lose. If you spam grabs, he can probably ftilt you out of your grab which is pretty stupid. If he's ftilting you, hold out your shield until he does his second hit and shield grab him. If he's good, he will mix up with 1st hit into a grab.
6) Don't get grabbed. Being as a golden rule for everyone, this is game changing snake vs. oli. If the snake has a quick reaction, olimar's roll sucks so he can tech chase you or shield grab if you normal get up attack. It's good to try to get up a few times, and then just sit there a second and then do something the next time. But, if you aren't careful he can just dthrow--utilt you. I like to get up by tapping up and doing a utilt, but they eventually catch on, and that's when u just get up normal they will shield for the utilt and you grab lawlzzzz
7) Edge guarding, if he is dumb and approaches you at low %, grab bthrow off the stage and jump fair them as they waste their double jump (nearly EVERY SNAKE DJUMPS IF YOU BTHROW OFF AT LOW %), but be careful they might jump nair, in which you can just grab them as they land on ground with a nair or smash their legs. If you expect them to jump nair in this scenario, you can space your fair and hit his bottom leg as his other legs spin up top, and it will completely go through his nair. Aim for the foot.
8) Snakes Nair---if the small possibility if you getting hit, you will most likely want to smash di INTO his body and come out the other side. Or di away if he is floating backwards while nairing. If you try to whistle it, spam whistle as you di and you can whistle every single hit of his nair with separate whistles
9) F-Smash is your friend-- You can fsmash him if he tries to mortar slide, you can fsmash his ftilt if you space it away from him, you can pretty much stop most of his stuff with an fsmash if he's not aerial, just be extremely cautious as you can fsmash his grenades and do damage to yourself and lose your momentum
10) Dthrow follow ups include standard dthrow grabs, pseudo grabs, and at low-med % if they di above you, dthrow and charge a dsmash or a fsmash, me and blackwaltz do this that i know of and snakes always air dodge right into it and it's free decent damage. Also, if you fsmash in this scenario if he is low enough you can dash grab him before he moves, as shown in dabuz vs. ally wifi match, granted into wifi, it's still a very nice follow up
11) I forgot to mention in recovery, if you jump towards the edge some snakes randomly throw out a dtilt and it goes slightly below the stage and WILL hit you. Just be careful for anything.
FINAL NOTE:: Camp him as hard as you can, and punish all of his everything with spacing, and watch out for grenades and you are good.
oh, i forgot to mention, if you are one of the oli's who likes to uair through the edge of the stage hitting them underneath, if he drops a nade it WILL stage spike you, and that's an instant stock.
As far as counterpick stages, i love bringing him to any neutral but battlefield, i love yoshis for the quick dsmash kills and fd for the camping, lylat is really good if you can keep track of his nades and mine with the shadowing effects. Neutrals are your friend in most matchups, so keep that in mind