I more often see oolong tea promoted as a weight-loss tea than green tea. However, studies I've read seem to suggest that this effect is due only to the caffeine content of the tea--as caffeine does have effects on weight gain and loss. This first article studies green tea and finds that while caffeine has an effect, the tea itself does not have any additional effect:
The following study essentially says the same thing for Oolong tea:
Yes, tea is very good for you and has many health benefits, both green tea and other varieties of tea...and yes, it does help you lose weight...but it may just be the caffeine is what is helping you lose weight, at least according to these studies.