To dance, usually by only moving one limb, like standing (nearly hopping) on one leg whilst kicking the other out in time to the music, sometimes with an arm pointing in the air, appearing to be pissed or falling over, Can only be done if you don't think you are cool whilst doing it. The word originated sometime 1997-98 and was mainly used for hard hitting drum and bass or what was to become 'speed garage', though the dance had been around for a few years longer. "give it some ooga flex" can also be a way of dancing to show you could't give a damn what people think. Some people even like(d) to ad the words Brap, Blap, Bo, Boo. (bo and boo came from the same kinda scene as the dance, you seen it?)
Could be seen done by many people at various jungle sessions from '94 onwards, then later done by crowds of people with one arm in the air pointing, showing respect for a tune in the garage scene which somehow evolved on to 'grime. The ooga flex dance would often be disguised in one move with a "Brap!!" at the same time.