There has been work done on how teaching mindfulness meditation breaks the 'OCD circuit' (the activation of the neurological pathway involved in acting on compulsive behaviours) as well.

Mindfulness meditation seemed to help people prevent the action and long term it seemed to help prevent the urge.

I'm not sure what studies have been done on mindfulness meditation for ADD and ADHD but I wouldn't be surprised if people who practice meditation regularly have an increased capacity to focus attention.