Bob Grant, playing bus conductor Jack, had actually worked in real life as a bus driver in order to pay his way through RADA (the Royal Academy of Dramatic Art). He mentions this in the TV interview he did on the Wogan show in 1990. The rest of the cast were also present talking about the planned new series which sadly never got made.
Reg Varney wouldn't sit out his part all the way. He left the series in the final season, after episode 69 (entitled "Goodbye Stan"). So Varney wasn't in the last six episodes of this series. His absence was explained by Stan moving to the Midlands, to work in a bus making factory. Inspector Blakey moved into the Butler household as a lodger, so he promoted from a bit-player to the show's primary character. Varney did feature in the third "On the Buses" movie later on in 1973, Holiday on the Buses (1973).
Michael Robbins (Arthur Rudge) left the series after season six to concentrate on his stage work again. The first episode of the final season is called "Olive's divorced", so Arthur and Olive finally get divorced.
Due to a technicians' strike during season four, seven of the thirteen episodes were made in black and white rather than colour - "Christmas Duty", "The Anniversary", "Cover-Up", "Safety First", "The Lodger", "The Injury" and "Not Tonight". On some of these a voice is audible directing the audience's applause prior to the credits cutting off.
As Olive, Anna Karen played a character who was presented as frumpy and uncaring of her appearance, though in real life, Karen had been a model and dancer prior to her breakthrough On the Buses role, and when she showcased her natural look during a magazine shoot in the 1970's, viewers were shocked at just how different Karen looked from her character.