O’Neill Thermo X Long Sleeve Rash Guard

O'Neill Thermo X Long Sleeve Rash Guard: Base-Layer Insulation for Wetsuit Wearers and Water Sports

The O'Neill Thermo X Long Sleeve Rash Guard is built specifically to be worn underneath a wetsuit, not just on its own at the surface.

If you are a recreational diver or surfer who wants an extra layer of warmth without adding bulk, this is worth a close look. If you are shopping for sun protection at the beach or on a snorkeling day trip, it works for that too, but the design priorities here are clearly aimed at the under-wetsuit use case.

What the Thermo X Fabric Actually Does

The shell is a 220g nylon/spandex blend treated with O'Neill's HyperDry finish. That treatment reduces how much water the fabric absorbs when you first submerge.

Why does that matter? A rash guard that soaks up water and holds it against your skin pulls heat away from your body. One that sheds water quickly keeps a thin layer of warmth between you and the inside of your wetsuit. The Thermo X is designed around that principle: maximum insulation for its weight, not maximum thickness.

The fabric is rated at less than 2mm thick, so it adds meaningful warmth without making it harder to pull your wetsuit on.

Fit Details That Matter Under a Wetsuit

Getting a rash guard to behave under a wetsuit is harder than it sounds. The Thermo X addresses the two most common frustrations:

• Low neckline: The collar sits well below where a wetsuit neck seal sits, so there is no bunching or pressure point at your throat.
• Boardshort connector: A small loop or tab at the hem attaches to your boardshorts or swim trunks, keeping the guard from riding up your torso as you pull your wetsuit on.

Both of these are practical details that cheaper rash guards skip. Ride-up under a wetsuit is genuinely annoying once you are in the water and cannot fix it.

Construction and Comfort

The seams are flatlock stitched, meaning the seam allowance lies flat against the fabric rather than folding over into a ridge. Under a wetsuit, any raised seam becomes a pressure point over a long dive or session. Flatlock construction removes that problem.

The 4-way stretch fabric moves with you in every direction. That matters both for donning your wetsuit without fighting the underlayer and for comfort during any activity where you are reaching, paddling, or kicking.

Surface and Standalone Use

The Thermo X does carry UV protection, moisture-wicking properties, and quick-dry performance, so it is usable for snorkeling, surfing, or general water sports on its own.

That said, its primary strengths are thermal and functional, not aesthetic. If standalone sun protection is your only goal, lighter and more affordable rash guards exist. This one earns its place by doing the under-wetsuit job well.

Who This Is Not For

This is a men's cut. If you are shopping for women's sizing, you will need a different model.

It is also not a substitute for a thicker wetsuit in genuinely cold water. The Thermo X adds insulation at the margins. It is a complement to your wetsuit, not a replacement for moving up in wetsuit thickness when conditions demand it.

Sizes and What Is in the Box

The Thermo X is available in multiple sizes. There is no zipper or other closure; it pulls on over the head. The boardshort connector is built in at the hem.