Expy: Savalas was the inspiration for Lex Luthor as portrayed in Superman: TAS. Like Blofeld, Luthor is a rich cretin with a thirst for respectability. Also, he's quick to drop the genteel act and throw down if necessary.
Draco is French (Corsican), and he's played by Italian actor Gabriele Ferzettinote though he's dubbed by David de Keyser, who would later have a small role in Diamonds Are Forever.
Irma Bunt is Swiss, Ilse Steppat was German.
Grunther: Swiss or German character, played by a Russian stuntman.
Magnum Opus Dissonance: Despite being one of the most obscure Bond films amongst the general populace, often inaccurately thought to be a financial failure, the producers and EON have gone on the record saying that if they had to choose one Bond movie to preserve for future generations, it would be OHMSS.
Irma Bunt was Ilse Steppat's only English language role. She died shortly after the film came out.
Peter Hunt drifted away from the franchise after this film. In an interview, he explained that Broccoli and Saltzman asked him to return for Diamonds Are Forever and subsequent films, but he was always busy with other projects at the time and sooner or later they stopped asking.
Telly Savalas replaced Donald Pleasance as Blofeld. This is a holdover from the original book, where Blofeld had plastic surgery to alter his appearance to make him harder to identify.
Small Name, Big Ego: George Lazenby was reported to be quite a prima donna on the set, sure that he would become just as big a star as Connery with the role. This probably contributed to his not doing any more Bond films.
Lazenby has gone on record as saying that it was mainly him listening to very bad advice by his agent that made him behave that way, including his decision to stop making Bond films. The fact he always said in later interviews that this was one of the worst decisions he ever made but that he also fired his agent just a couple of years later really does not come as a surprise if this in fact is true.
Throw It In: The St. Bernard rolling in front of Bond was totally unexpected, and Lazenby reacted accordingly ("Never mind that! Go and get the brandy, huh?").
Troubled Production: Granted, not anywhere near as much as the press at the time made it out to be, but there were a lot of disagreements between Lazenby and director Peter Hunt, and also between Hunt and the producers, which resulted in neither man ever returning to the series.
The Incredible World of 007, a retrospective book of the series up to Licence To Kill, mentions that Lazenby and Diana Rigg did not get along during filming.
The role of Bond was offered to Timothy Dalton, who declined because he felt he was too young and inexperienced, and Adam West, who declined because he felt Bond should be played by a Briton.
In the commentary, George Baker (Sir Hillary Bray) explains that Ian Fleming had put him forward as his choice to play James Bond when he was trying to drum up interest for a film series, before Saltzman and Broccoli got involved. In that light, Baker noted it gave him an odd feeling to be in the studio dubbing Lazenby's dialogue for when Bond was impersonating Bray.
Had Lazenby's agent not convinced him to leave the series, his contract was for seven movies. Tracy's death would only occur in the follow-up, but since the main actor was leaving, the producers decided to keep it in just one movie—and Bond seeking revenge for Tracy's death was not addressed within the franchise until For Your Eyes Only.
Irma Bunt was supposed to come back in the next film (likely for Bond to get a proper revenge on her), but Ilse Steppat died of a heart attack only four days after the international release of OHMSS, hence the Karma Houdini.
In order to explain why Bond looks different, the original script explained that he had plastic surgery because he kept getting recognised by all his enemies. This was scrapped because they realised it was a terrible idea.
An entire action scene was cut due to time. Bond was originally supposed to catch one of Blofeld's henchmen spying on his meeting with Sir Hilary and give chase across the rooftops of London.