Hi !
Micromart carries a line of HO scale rivet decals that I've used on 1/72. They might carry an "O" gauge assortment, I just don't remember....you can check them on line.
I cut these to length and then apply just as I would any decal, let dry, cover with matte, then paint as needed.
Someone else mentioned "ponce wheels" these work nice for flush riviting and the come with different diameter wheels that simulate other sizes/scales.
Finally, you could drill a series of "small holes" along the panel line and then fill the hole with stretched sprue....let it dry completely and then sand......I've never tried this but after the sprue is glued and set you might be able to "round the head" by bringing a hot pin close to the sprue.......like you do when you use sprue for flying wires and need to bring it taught....