OHVA is the "Off-Hook Voice Announce" feature.

Think of it like an enhanced "call waiting" feature.

During an in-progress call, a user recieves a brief voice announcement advising them information about a second incoming call. The user can accept or reject the second call by pressing a designated button.

It is also explained in this DCS Brochure:
https://www.telephoneconnection.com/PDF/DCS_broch.pdf