In my opinion, the matchup is pretty even, maybe with a slight advantage to oli. I played a good oli for awhile about a month ago. Here is what I remember from the matchup.
Olimar out-camps snake by far. I lost every match where I tried to camp him back. In the time it takes to cook a grenade I would have mulitple pikmin thrown on me and Oli had plenty of time to predict and shield the grenade's explosion.
Since camping didn't work out too well, I then went to approaching with mortarslides. Then he would just smash me out of it since the mortarslide has very little priority. Or he would time the grab and combo me from there. Another approach I used that worked better was to just run towards him and when I got within grab/smash range I would shield or spotdodge depending on whether he tried grabbing or attacking. After he would make his move I would ftilt or grab.
Snake seemed to beat Oli in the air unless Oli was coming from below because snake's dair isn't usually effective. Also, once off the edge Oli is pretty easy to kill. If he was close enough to get back without an upb I would jump off and nair or bair him to his death.
In order for snake to win, I had to: Play aggresive and keep him as close as I could so he didn't have an opportunity to pikmin camp, avoid his grabs, and try to get him off the edge. I would lose when I let him camp. So IMO for Oli to win he needs to play defensively and camp until snake is forced to approach and then react appropriately.
This might not be completely accurate since it was over a month ago and that's all from memory, but that is what I remember from it. Hope it helps.
PS. This thread helped me a lot in a tourney on saturday. A couple pages back it talked about how luigi's mansion is a good stage for Oli and a sonic player counterpicked that stage against me in the finals since it sucks for snake. Then I remembered this thread
It's the third match in this playlist. (youtube is blocked where I am so I can't pull up the exact link, sorry) Feel free to critique my oli, I would appreciate it.
http://www.youtube.com/watch?v=ZXhp7...x=0&playnext=1