In small- and medium-sized enterprises (SMEs), where typically the decision-making process is highly centralised, important decisions, such as open innovation (OI) adoption, will be strongly influenced by the characteristics of their Chief Executive Officers (CEOs). Pointing the attention to the strategic leadership and human elements, this paper sheds light on the micro-foundation of OI by emphasising the role that the personal traits of key individuals in innovation. OI adoption could result in the enactment of several OI modes - each representing an opportunity of potential change (of market, of technology or/and of the organisation) - and this paper attempts to examine the relationships between the CEO characteristics and each of the OI modes. Our analysis, using Korean SME data, shows that CEOs' positive attitude, entrepreneurial orientation (EO), patience and education can play important roles in facilitating OI in SMEs. However, this paper also observed that the effects of CEO characteristics on OI adoption were differently configured according to the nature of each OI mode, for example, CEOs' patience and EO had different impacts depending on the degree of uncertainty in the OI mode. This suggest that OI must be understood as a wide innovation spectrum, and, to increase opportunities for successful OI adoption, CEOs have to attempt to compensate for characteristics they may lack by recruiting appropriate complementary top managements. The research has practical implications for CEOs and policy makers who are interested in enhancing competitiveness of SMEs.