ΔE 7.8, close: similar, with a visible difference side by side. Abaddon Black is a little darker than Outlining Black.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Abaddon Black is a little darker than Outlining Black.
The same difference applies the other way round: Outlining Black is a little lighter.
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 Outlining Black on the colour wheel See Abaddon Black on the colour wheel
All matches: Outlining Black equivalents · Abaddon Black equivalents