Online Behavioral Advertising (OBA), which involves tracking people's online behaviors, raises serious privacy concerns. We present results from a scenario-based online survey study on American and Chinese Internet users' privacy preferences of OBA. Since privacy is context-dependent, we investigated the effects of country (US vs. China), activity (e.g., online shopping vs. online banking), and platform (desktop/laptop vs. mobile app) on people's willingness to share their information for OBA.We found that American respondents were significantly less willing to share their data and had more specific concerns than their Chinese counterparts. We situate these differences in the broader historical, legal, and social scenes of these countries. We also found that respondents' OBA preferences varied significantly across different online activities, suggesting the potential of contextaware privacy tools for OBA. However, we did not find a significant effect of platform on people's OBA preferences. Lastly, we discuss design implications for privacy tools.