The ending of On Her Majesty's Secret Service is that Bond marries Tracy in Portugal, and Blofeld and Irma Bunt shoot at their car as they drive away, killing Tracy while Blofeld stays free.
First, an avalanche splits Bond and Tracy and leaves her in Blofeld's hands. Second, Draco and his men join Bond for the raid that wrecks the mountain base. Third, Blofeld escapes on a bobsled and is caught in a tree branch after a crash. Fourth, the story jumps to Portugal for the wedding. Fifth, Blofeld and Irma Bunt fire on the couple's car as they drive away from the ceremony.
So, why does it matter? The raid and the bobsled give the audience the satisfying action finish, and then the film takes it away. Bond has stopped the plague plot and wrecked the base, yet he ends the film holding Tracy's body and refusing to accept that she is gone. In my opinion, that choice is the strongest in the film, because it makes the villain's escape feel personal rather than procedural.
The setup matters as much as the shooting. Bond first meets Tracy when he stops her drowning herself, he proposes in a barn after she helps him escape Piz Gloria on skis, and the wedding follows within the same story. The film spends its running time of 142 minutes on that relationship, which is why the final road scene in Arrábida National Park lands as a loss and not as a twist. Tracy is also the only woman Bond marries on screen in the official series, so the death cannot be quietly forgotten by the next film.
Compared with the five films before it in release order, from Dr. No to You Only Live Twice, this is the first Bond story to end on a defeat for Bond after the villain's base has been destroyed. Blofeld loses his clinic and his plot, yet he walks away with the last move. That is an opinion about structure, not a claim about every earlier ending, but the contrast is clear: the raid on the mountain is the conventional climax, and the Portuguese road is the real one.
Because Blofeld is still free, the feud stays open for later films, and the next film in release order is Diamonds Are Forever. Paradoxically, a film about a wedding is the one that leaves Bond with the most unfinished business. For how the later films use Blofeld, the james bond villains page is the next stop, and the james bond movies ranked page shows where we place this one.