What paint brands are you using? If you're using Pactra, their black is a blue based pigment. For the most part, Fasblack is also a blue based pigment.
For a decent OD green, use non-glow or flourescent paints, and no metallics. Take a basic yellow, about 7 drops, add one drip black and mix thoroughly. If it comes out too dark, add a few drops of yellow. Next, add a drop of a non-metallic/flo/glow medium blue. You should wind up with a baby turd green. If you need to lighten it up, add a drop of white to pale it some. If it's too light, add a drop of fastint. OD Green will never come out quite right because you're painting on the inside which makes it look glossy. Hope this helps.
Before mixing in a jar or in your airbrush, mix on some plastic wrap to fine tune the formula and keep good notes on it to make it easier to reproduce in your airbrush or in a jar.