How should I fix a gap in this mitered corner?

If the joint isn’t glued yet, I’d fix the cut rather than try to disguise a sizeable gap. Check the saw’s angle against a reliable square, then make a test cut on scrap and bring the pieces together before trimming the oak plywood. A small adjustment may be enough. Avoid sanding much off the faces, since the surface veneer can be thin.

A support block can strengthen the corner, but it won’t close the visible seam. If a slight gap remains after fitting, a thin sliver of matching material will usually blend better than a wide smear of filler.

For a gap you can see through, don’t count on glue mixed with sawdust to make it disappear. That mix can look noticeably different from the plywood veneer, and it’s better suited to tiny imperfections than a large opening. If you can’t recut without losing too much material, fit a narrow piece of matching wood into the gap, glue it, then trim it flush carefully.

If the corner is on the underside or will be covered, it may not need cosmetic repair at all. If it’s exposed, though, a clean recut is the tidier fix when there’s enough material. Make a scrap test first, and aim for a fit that closes on the visible outside of the corner.