(Update: after reading the Wikipedia article I see that it probably refers to "his colleague", but I wasn’t familiar with the use of "oppo" here.)

https://www.hmsrichmond.org/dict_o.htm https://en.wiktionary.org/wiki/Appendix:Australian_English_m...

> OPPO > Naval slang - short for opposite number - for the person doing the same job as one's self in another watch or ship. In the former case, since you relieved each other (in the old two-watch days), it behoved you to become friends; thus the word Oppo came to mean Chum. On a two-watch watchbill, the name of a man's 'opposite number' in the other watch was shewn against his own name in the opposite column.

I see that this is a British English word, so it brings to mind the old saying: "Two peoples divided by a common language."

Was pretty common to hear amongst the wartime generation, but has declined pretty rapidly since the 80s. Never once heard it used for "sweetheart" as Webster's seems to think.