Oliver Reed's voice is not heard; he's dubbed throughout by another (anonymous) actor attempting to sound like Reed's normal voice. Reed's own voice is heard in the film's trailer, where he adopts a very camp lisping French accent.
Richard Greene was the only actor to reprise his role from the TV series. The other roles were played by an entirely new set of actors.
The movie as we now have it is without its original opening sequence, which was cut; apparently, it depicted the Lord being murdered while on crusade and his squire fleeing to Sherwood Forest on his horse. As it is, the movie now opens with him riding into the forest on horseback and his identity is never revealed.