the little o means it is a diminished chord. I've got it, thanks.
Tom
actually that's incorrect, if as you said before there is also '7'.
That means it's a DIMINISHED SEVENTH chord. It looks similar but in fact it's not really....it has a diminished seventh on top beside the diminished fifth.
A diminished triad will work too, but the harmony will be incomplete and not intended as on the sheet.