ABSTRACT This article comparatively examines the efficacy of opposition political parties in constraining democratic impunity and advancing development in Nigeria and South Africa. It proceeds from the proposition that opposition effectiveness cannot be reduced to alternation in executive office. Rather, efficacy includes the capacity to impose electoral costs, activate legislative and judicial accountability, expose abuse, mobilise citizens, influence policy and compel governing elites to justify or reverse decisions. Using a qualitative comparative case-study design, the article analyses constitutional rules, electoral outcomes, parliamentary practice, leading judicial interventions, governance reports and relevant scholarship from the democratic periods of both countries, with particular attention to Nigeria since 1999 and South Africa since 1994. The analysis finds that Nigeria demonstrates the power of opposition coalition-building through the 2015 transfer of presidential power, but also the fragility of that power when opposition parties become fragmented, personalised and vulnerable to defections. South Africa, by contrast, illustrates how opposition can be consequential without immediate national alternation: parliamentary contestation, constitutional litigation, subnational governance and sustained electoral erosion of African National Congress dominance culminated in the 2024 loss of its parliamentary majority and a Government of National Unity. The article argues that democratic impunity declines where opposition strength interacts with credible institutions, civic scrutiny and enforceable legal constraints. Developmental gains are therefore indirect rather than automatic. Stronger programmatic parties, more credible electoral administration, disciplined legislative opposition, transparent political finance and institutional protection against opportunistic party switching are recommended. Keywords: Accountability; Democracy; Development; Impunity; Opposition parties