Gravimeter measures gravitational acceleration, which is valuable for geophysical applications such as hazard forecasting and prospecting. Gravimeters have historically been large and expensive instruments. Micro-Electro-Mechanical-System gravimeters feature small size and low cost through scaling and integration, which may allow large-scale deployment. However, current Micro-Electro-Mechanical-System gravimeters face challenges in achieving ultra-high sensitivity under fabrication tolerance and limited size. Here, we demonstrate a μGal-level Micro-Opto-Electro-Mechanical-System gravimeter by combining a freeform anti-spring design and an optical readout. A multi-stage algorithmic design approach is proposed to achieve high acceleration sensitivity without making high-aspect ratio springs. An optical grating-based readout is integrated, offering pm-level displacement sensitivity. Measurements reveal that the chip-scale sensing unit achieves a resonant frequency of 1.71 Hz and acceleration-displacement sensitivity of over 95 μm/Gal with an etching aspect ratio of smaller than 400:30. The benchmark with a commercial gravimeter PET demonstrates a self-noise of 1.1 μGal Hz