The structural complexity of agroforests is vital to high ecosystem function, stability, and resilience, but its development has rarely been quantified. Here, we present results from a large-scale restoration experiment, EFForTS-BEE, conducted in an oil palm plantation in Sumatra, Indonesia. In the experiment, six native tree species were planted in a conventional monoculture oil palm plantation, systematically varying plot size (25–1600 m²) and initial tree diversity (0–6, 0 corresponding to natural regeneration only) by following a 40 % oil palm thinning in 2013. Using a stand structural complexity index (SSCI) derived from terrestrial laser scanning data, we examined the nine-year development of structural complexity of the experimental oil palm agroforests. We computed the SSCI considering the vegetation visible from the scanner at the center of the plot. Tree-planting plots showed the development of denser and more complex structures towards reference primary (logged) forests, compared to no-planting (initial tree diversity of 0) plots and the conventional oil palm plantation. In nine years, tree-planting plots, on average, showed 40 % higher SSCI than the conventional oil palm plantation and 67 % SSCI compared to reference forests. In tree planting plots, initial planted tree diversity and plot size did not significantly affect the development of the SSCI. After nine years, planted tree species showed varying mortality rates, and the basal area of naturally, regenerated woody plants was positively correlated with the SSCI. On the other hand, the basal area of planted trees was negatively correlated with canopy openness, and oil palm height was positively correlated with canopy height and vertical stratification. We showed that planting trees increased structural complexity in the conventional oil palm plantation during early succession. However, as naturally, regenerated woody plants seemed to drive the SSCI nine years after tree planting, it is crucial to allow natural regeneration to increase structural complexity for a long time and consider the temporal dynamics of vegetation to manage structural complexity. As the SSCI did not depend on plot size, even small tree patches can increase structural complexity locally in an oil palm plantation.