Self-transcendence and the associated decrease in ego-centeredness are understood to foster altruistic and positive behaviors that promote prosociality and pro-environmental actions. However, the lack of an agreed valid and reliable measure of self-transcendence has posed difficulties in examining this subject. The main aim of this study was to use the recently developed Ontological Addiction Scale (OAS), which assesses dysfunctional ego-centeredness as an inhibiter of self-transcendent change, as a proxy measure to examine whether self-transcendence is indeed related to such “selfless” prosocial and pro-nature behaviors. Secondary aims were to examine ontological addiction (OA) and these behaviors in relation to competitiveness and ego-driven manifestations of prosociality. These relationships were examined via self-report measures in a cross-sectional cohort study of 287 UK adult participants. Findings revealed associations with competitiveness that broadly reflect its association with ego-centeredness through significant correlations with OA, and significant negative correlations with both prosociality and pro-nature behavior. They also supported earlier findings of a significant relationship between OA and measures of anxiety and depression. However, the expected negative correlations between OA and both prosociality and pro-nature behavior were not evidenced, which raises questions regarding the notion of OA as a conceptual antithesis of self-transcendence and/or whether measures of pro-sociality and pro-nature behavior adequately account for “selfish” or “impure” motivations that may underly these behaviors. Implications and future research directions are discussed.