Watch the Flanger Before OD online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide
Old school amps like the ones Hendrix and Van Halen were using did not have an effects loop so pedals all went before the amp. This is very important to understand when we are trying to recreate some of those classic tones with pedals. In this case the OD pedal is acting as the distorted amp so I would put the flange in front of OD. This is what you hear Van Halen doing on tunes like "Unchained", he is using his cranked Marshall with a flanger in front of it. If you want that sound, you must put the pedals in that order. Personally, I much prefer this FX chain, largely because I love those classic tones.