As a major component of Earth’s energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in the Earth’s energy budget. Here we investigate the annual cycle of the upper 2000 m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence and meridional heat transport. Results show that the robust signal of annual OHC change is significant down to 1000 m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0-1500 m) changes from positive anomalies within September-February to negative anomalies within March-August mainly because of the larger ocean area in the southern hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycle in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport is the main driver.