How do you render OSA in the Sequence editor ?

I am trying to use zutility to render out zdepth but can’t get it to anti alias at all, even with OSA cranked up to 16

A zbuffer is not an image, you can’t really anti-alias it. it wouldn’t really make much sense. Unless you mean a zbuf-like compressed grayscale type of image, in which case then just the old method of rendering at higher resolution and resizing might work best.
Or maybe try creating a fake depth image using mist/fog. It depends on what you need it for.