I would have thought it stood for one size, but after looking online I can't find any reference to it.
It could mean size zero.
Size 0x is the smallest size in the run of plus sizes 0x, 1x, 2x and 3x. It is roughly equivalent in size to a Misses X-Large but like most women's or plus sizes is cut a little differently to fit a "fuller" figure. I think the women's size equivalent would be a 16W.
The sizes XL and OX are in fact different. An OX provides more room in various areas of the garment such as the body, armholes, sleeve length and shoulders. And a 1X have even more room then the OX.
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