Green tea, as well as Oolong tea & Black tea, are produced from the herb Camellia Sinensis. They all contain the same amount of caffeine. Caffeine content in a cup of tea is 2.5% to 4%, which is about a third of that in coffee. It is claimed that 80% of the caffeine in tea remains unabsorbed by the human body.
From the above you would realise that Green tea, Oolong tea & Black tea may taste different but the caffeine content is the same.