I am not going to discuss if oolong tea can really helps you lose weight as results are not definite with only few research reports available in public and there are no conclusive results. Some people do and some people don't after trying oolong tea. Also, I believe controlling your diet is equally important.
Regarding"real authentic oolong", you must first be clear that oolong/wulong tea is just a category of tea classification (just like green tea,white tea etc). As long as the fermentation level is around 40-50%, it is an oolong /wulong tea. The only difference between oolong tea is due to the different processing method like pan frying etc, resulting in various grades of oolong/wulong tea like the premium oolong tea (Ti Kuan Yin), wuyi rock tea or Shui Sian and Tong Ding Wu Long etc. The basic properties are the same but more important to tea drinkers, it is the taste and aroma of the tea that classify whether one oolong tea is better than others. Not its "potential slimming properties" So, if u can get real Natural oolong tea (not green tea, white tea, black tea etc) from your local store, just go for it and try it out. There is no harm in drinking it. Just make sure its not those "slimming tea" that may have added additives to it - may have side effects and to me it is not a natural tea.
You are welcome to drop by my blog for more info on oolong/wulong tea.