Many breweries bottle with a different yeast than what they brew with (for better carbonation control? To keep their strain a secret?). That Oberon yeast may not give you what you're looking for. Just sayin'. I wouldn't put it past Bells.
If you didn't want to take a chance, then you'd be best starting with fresh built-up liquid yeast, like White Labs American Hefe. Two quarts of 1.040 with light DME should do it, though you may want to use wheat DME if you're going to dump this into a wheat ale.
As for what to do to build up that Oberon cake... You're making a starter from a very small amount of yeast. You don't know how old that yeast is (though it's probably pretty dang fresh, as Oberon just hit the shelves a couple of weeks ago), so it's hard to say how well a certain amount of cake will perform. Don't worry about that small amount of beer, though if you want to be certain, you can settle it in the fridge usually overnight. Carefully dump it off, but not all, as you'll want a small amount to resuspend the yeast with.
If I were doing this, I'd start it in a quart of light DME at about 1.040 or slightly less. Keep shaking the daylights out of it for a day or two. Boil up another two quarts, and drop it into the first batch. Keep shaking it for a day or two. Toss it into the fridge and settle it; a day should work.
Check the amount of cake; should have at least a cup in there. If not, build it up one more time with another two quarts of DME wort (dump off the last wort first). That should be more than enough cake to make a wheat ale of 5.5-6% ABV.
EDIT: I should mention that it's not a good idea to use too much yeast. If you've got a cup's worth, that'll be fine for a 5-6 gallon batch. Beyond that, you could keep some in the fridge for up to a month (for more info, look around for posts on yeast farming). I'd see what the yeast does first, though.