The A23 OC rim is the off center version of the popular A23. Off center design allows for dramatically improved spoke tension on a dished wheel.

This is what Velocity says:

With a wider footprint, the A23 allows for a more aerodynamic rim/tire combination when paired with a 23c tire. The tire profile mimics the characteristics of a tubular, gaining increased control and lower rolling resistance without the inconvenience of tubular glue or the dangers of irreparable roadside mechanicals. A tubeless ready design makes this rim ideal for road and cyclocross applications.

Tight tolerances require the use of a low profile rim tape like Velotape to avoid tire installation issues.