ΔE 8.2, close: similar, with a visible difference side by side. Flat Brown is a little lighter, more saturated, cooler than Oxide Brown Dark.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Brown is a little lighter, more saturated, cooler than Oxide Brown Dark.
The same difference applies the other way round: Oxide Brown Dark is a little darker, more muted, warmer.
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 Oxide Brown Dark on the colour wheel See Flat Brown on the colour wheel
All matches: Oxide Brown Dark equivalents · Flat Brown equivalents