I usually wear a jersey or t-shirt under the pressure suit, but it can be worn with nothing under or over. I also wear the Ogio flight vest (love it!) which goes over the pressure suit. Or, a jacket over the pressure suit when its chilly and the vest (or a back pack) over.

If you get the AXO, be sure to order large as Atomic suggests. BTW, I bought the items mentioned (AXO pressure suit and Klim Baja pants) from Atomic. Their prices were great and they treated me just fine. I'd order from them again. (no affiliation with them...just a satisfied customer.) Looking back on it though, I think I'd get a suit with a full breast cover instead of the two part on the AXO, as it leaves the sternum a bit exposed.