Vo (operating manoeuvring speed) was introduced at A/L 23-45 in 1993 ref
FAR Part 23 Sec. 23.1507 effective as of 09/07/1993
AC 23-19A (
https://www.faa.gov/documentLibrary/.../AC_23-19A.pdf) has this to say -
48. What is the design maneuvering speed VA ?
a. The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs√n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in Part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA= Vs √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507. VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.