I never used onesies with DS. Particularly since we had him in wool, and that's just asking for compression wicking. His entire wardrobe was tee shirts, sweaters, and the like, with soakers, shorties and longies for the bottoms, and plenty of babylegs. You have to look, but tees can be found. Also, if you're going to have a shower, I'd ask the organizer to pass that info along to people, since people seem to love giving onesies.
At night-time, we used a wool sleep sack and a tee shirt. I was co-sleeping, and that was plenty even in the dead of winter.