None of the research regarding the issue from OEM-supply to OBM has been paid to exploring the nature of competitive dynamic and coopetition between an OEM supplier and its brand partner. The OBM is considered as “initiating competition in cooperation”. How can an OEM supplier move to reduce the chance of retaliation or to delay opponent’s response? How the competitive dynamics perspective may apply to interpreting the phenomenon of initiating competition in cooperation? This study conducted a multiple-case method to investigate four companies, demonstrating that an OEM supplier would consider both competition and cooperation strategies. It may proactively reveal its branding intention to increase partner’s awareness. It may avoid head-on competition and decrease the market commonality to diminish the partner’s response motivation. It may commit resource investment to increase the resource similarity, thus escalating the partner’s difficulty of responding. Theoretically, this study explores phenomenon of “initiating competition in cooperation”, which is different from the “conventional competition” and “cooperation with competitor”. This study contributes to the extension and application of both competitive dynamics and coopetition perspectives. Practically, the findings and propositions derived from this study provide important references for managers’ strategies moving from OEM-supply to OBM.