The RSO is the regional security office of the US Embassy in Beijing. Via the Oriental-List:

RSO has confirmed that in early April local police conducted a raid in the bar district of Sanlitun. More than 20 people were detained by police, including at least eight foreigners and, although the target of the raid appears to have been illicit drug dealers and users, some patrons in the area were temporarily detained. The Beijing public security authorities have reported that they closed two bars in the area and seized a variety of illegal drugs. RSO issuing a separate report on nightclubs and bars in Beijing.

I hear the RSO parties like crazy. Now that's one Beijing guide I'd like to get my hands on!