Thought it time to "get them in" - bit of a guess mind you - my Oca is still going great guns
Yep, that looks like just the right time to harvest, a harsh frost can harm the tubers. Very good sized plants and tubers from such small sad looking lumps I sent. They do seem to respond well to some good soil.
Don't forget to safely store the small growing tubers near the base of the plants for next year, they'll probably do even better, if you fancy growing them again.
Have you tried cooking them, they tend to stay crunchy for ages.