ΔE 20.7, loose: different enough that you should pick on purpose. Old Gold is noticeably lighter, much more saturated than Old Brass.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Old Gold is noticeably lighter, much more saturated than Old Brass.
The same difference applies the other way round: Old Brass is noticeably darker, much more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Old Brass on the colour wheel See Old Gold on the colour wheel
All matches: Old Brass equivalents · Old Gold equivalents