Theoretically, one of the ways governments that aim to improve economic growth in their countries is by increasing expenditure on human capital. However, some empirical evidence from OIC countries do not support the finding that expenditure on the two most important aspects of human capital (expenditure on education and health) affects growth positively. One of the plausible reasons for the observed unusual finding could be the presence of moderating variables. Therefore, this study investigates the relevance and applicability of one such moderating variable, namely the quality of institutions, given the relatively low global ranking of OIC countries in terms of institutional quality. This study contributes to the new institutional economics literature by discovering that some institutional quality variables determine the way human capital expenditure affects economic growth. This study uses the methodology of panel data analysis and interaction graph plots. Interaction graph plots show the marginal effect of a continuous variable on another continuous variable. Without the inclusion of interaction terms, we find that, on average, in OIC countries, the effect of government education expenditure (GEE) on growth is insignificant while the effect of government health expenditure (GHE) on growth is either insignificant or significant and negative. The interaction graph plots show the effect of government human capital expenditure on growth at different levels of growth. When we consider the marginal effect of institutional quality, we find that generally, better bureaucracy quality, control of corruption, government effectiveness, law & order and rule of law augment the positive effect of GEE on growth. We also find that better government effectiveness and law & order augment the positive effect of GHE on growth.