Yes, onesie is in the scrabble dictionary

...and is worth 7 points by itself if you played it.

Onesie Definition

noun

A one-piece garment for an infant or small child, generally worn over a diaper.
Source: Wiktionary
A loose-fitting one-piece leisure garment covering the torso and legs:.
A one-piece close-fitting lightweight garment worn by a young child, usually having sleeves but leaving the legs uncovered and fastening with press studs at the crotch.