I know next to nothing about ABC but I downloaded an ABC file of 'Det var en lørdag aften' to play with. Using abcm2ps-8.13.16 (2017-10-24) on Linux.

It contained '\o' for ø but abcm2ps displayed just o

If I replaced it with the character ø - that worked.

I found a document here (pdf)which gave '\/o' for 'ø' and that worked too.

'\aa' works for å