Vo -- operating manoeuvring speed. Similar to Va.
Current references to look at include
(a)
FAR Final Rule Making - Small Airplane Airworthiness Review Program Amendment No. 4 (and search for Vo),
(b)
FAR 23.1507 (which is where it is defined), and
(c)
FAR 23.1583 (where it is required to be published if determined).
Just how relevant these references are to your particular POH I'm not sure .. but they give you a starting point to research the matter.
.. I gotta get a life ...