A center from the family of ???fourfold coordinated (FFC) defects???, previously predicted theoretically, has been experimentally identified in crystalline silicon. It is shown that the trivacancy (V3) in Si is a bistable center in the neutral charge state, with a FFC configuration lower in energy than the (110) planar one. V3 in the planar configuration gives rise to two acceptor levels at 0.36 and 0.46 eV below the conduction band edge (Ec) in the gap, while in the FFC configuration it has trigonal symmetry and an acceptor level at Ec???0.075???eV. From annealing experiments in oxygen-rich samples, we also conclude that O atoms are efficient traps for mobile V3 centers. Their interaction results in the formation of V3O complexes with the first and second acceptor levels at Ec???0.46???eV and Ec???0.34???eV. The overall picture, including structural details, relative stability, and electrical levels, is accompanied and supported by ab initio modeling studies.