ΔE 19.7, loose: different enough that you should pick on purpose. Poison Green is noticeably darker, warmer than Olive Drab.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Poison Green is noticeably darker, warmer than Olive Drab.
The same difference applies the other way round: Olive Drab is noticeably lighter, cooler.
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 Olive Drab on the colour wheel See Poison Green on the colour wheel
All matches: Olive Drab equivalents · Poison Green equivalents