If use a OD pedal on a already driven amp will it add to the amp distortion or will it do nothing?
Depends how you use it. In most cases, OD is used to boost amp distortion with gain knob set to minimum and volume knob set to max. It adds a little bit more distortion, but that's not the point of boosting an amp, people do it because it tightens and compresses the sound and makes it cut through the mix better.

Setting the gain knob on OD higher would give you more distortion, which could be useful as boost for solo or to increase overall gain.
It adds a fair bit more distortion, I'd have said (depending on how the amp is set). But other than that, I agree.
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If use a OD pedal on a already driven amp will it add to the amp distortion or will it do nothing?

it will definitely do something.

people usually run an OD in front of an 'already driven amp' to push the preamp harder. this usually results in more distortion or at least a different sound to the distortion.
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anything that pushes the preamp harder will result in distortion. how much is a different story. it can be subtle or if driven really hard really up the distortion level.