score:11
An online search for "O/R" turned up a list of mostly irrelevant meanings, but one meaning seems likely to fit here: on request. It's possible that your parents asked for the stamps.
In a comment, you ask
Is there any practical reason they might want the stamp, other than as a souvenir?
They might have thought that your presence in the UK had a bearing -- or might possibly have a future bearing -- on your right to retain British citizenship. Maybe they asked for stamps in their own passports to serve as evidence of their presence in the UK for reasons of taxation in the place where they were living at the time, and the officer stamped all of the passports, including yours.