Nanomechanical mass spectrometry has proven to be well suited for the analysis of high mass species such as viruses. Still,- the use of one-dimensional devices such as vibrating beams forces a trade-off between analysis time and mass resolution. Complex readout schemes are also required to simultaneously monitor multiple resonance modes,- which degrades resolution. These issues restrict nanomechanical MS to specific species. We demonstrate here single-particle mass spectrometry with nano-optomechanical resonators fabricated with a Very Large Scale Integration process. The unique motion sensitivity of optomechanics allows designs that are impervious to particle position,- stiffness or shape,- opening the way to the analysis of large aspect ratio biological objects of great significance such as viruses with a tail or fibrils. Compared to top-down beam resonators with electrical read-out and state-of-the-art mass resolution,- we show a three-fold improvement in capture area with no resolution degradation,- despite the use of a single resonance mode. © 2020,- The Author(s).