If you live in a super warm climate, you can skip this part of the article. The lucky ducks who want to beat the heat in Phoenix, Honolulu, or Miami or San Juan have never had to worry about staying warm in onesies. If you’re a warm-weather onesie wearer, you can throw on a low-profile pair of underwear (or whatever you’re into), and you’re good to go. Stick to underwear in the same tone as your onesie, so it doesn’t show through. For example, if your onesie is a light color, avoid black or brightly colored underwear.
For the folks in the chilly climes in places like Syracuse, St. Paul, or Salt Lake City, warmth is going to be a real concern. While hanging out at home doesn’t require a lot of undergarments, for people who are planning to go out in chilly weather, you should consider wearing something warm under your onesie. Be sure to choose something that covers your arms and legs, such as a long-sleeved t-shirt or leggings. Make sure the garments you choose to wear under your onesie are super close fitting, so they don’t bulk up your onesie. And it’s a good idea to avoid buttons or zippers since they’ll also make your onesie look bumpy. Thermal underwear is a good choice. It’s warm, close-fitting, and shouldn’t be visible under your onesie. If you have stretchy leggings, jogging pants or compression shirts, they’re also an excellent first layer to wear under onesies.