This study explores the adoption of open innovation (OI) practices in medium-sized and large firms in a sector characterised by low levels of external collaborations. Many firms struggle to adopt OI practices (O'Connell, 2011); the processes that lead to the adoption of OI practices are unclear (Mortara and Minshall, 2011); and the degree of open innovation, as measured by the number of external collaborations, in Irish firms is low (Vahter et al., 2012). This inductive study is based on case studies of a significant innovation in four medium-sized (€50m to €500m) and four large (Revenue above €500m) firms from the food sector in Ireland. In each of the firms, multiple senior managers (CEOs, innovation managers and marketing managers) were interviewed about the origin of the innovative idea; the management of the innovation; and the role of external partners and customers in the innovation process. Within and cross case analysis finds that the adoption of OI innovation practices are most common at the early stage of the innovation value chain (IVC); that managerial perceptions of competitive threats appear to limit the extent to which firms adopt OI practices at the conversion stage of the IVC; that at the diffusion stage OI practices are largely limited to collaborations with customers; and managers regard external interactions for market orientation as being open in their innovation processes. In terms of the process of adoption, the smaller firms in this study are characterised by ad-hoc adoption of OI practices, while in the larger firms there is some evidence of more ‘conscious adoption’ of OI practices (Mortara and Minshall, 2011). Contributions include an argument that OI practices differ by stages of the innovation process; that managerial perceptions limit the adoption of OI practices; that market orientation may be regarded as a subset of open innovation; and the development of emerging work that explores the adoption of OI in non-‘high-tech’ contexts.