On 15 January 2022, a massive underwater eruption occurred at the Hunga Tonga-Hunga Ha’apai Volcano. The plume reached the mesosphere, and the eruption excited a significant atmospheric Lamb wave, which forced the tsunami. The complicated tsunami waveforms due to ocean-atmosphere coupling prevented inferring the force history of the excitation. To address this, we analyze ocean surface gravity waves (OSWs) from 15 to 40 mHz, which are decoupled from the Lamb wave due to their slower phase velocities. Modeling these OSWs, we infer that the excitation started at 4:00 UTC with an amplitude of 1010 N and lasted for 5 hours, followed by a sub-event at 8:40 UTC. The observations suggest an initial blowout of seawater above the summit and a subsequent outflow that excited a tsunami below 5 mHz. The 2-hour delayed OSW excitation from 6 to 15 mHz may indicate seawater inflow into the crater.