ΔE 42.3, loose: different enough that you should pick on purpose. Oblivion Black Wash is much more muted than Kroak Green.
Can I use Oblivion Black Wash instead of Kroak Green?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Oblivion Black Wash is much more muted than Kroak Green.
Can I use Kroak Green instead of Oblivion Black Wash?
The same difference applies the other way round: Kroak Green is much more saturated.
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.