A first-principles study of the BO2 complex in B-doped Czochralski Si reveals a defect-bistability-mediated carrier recombination mechanism, which contrasts with the standard fixed-level Shockley-Read-Hall model of recombination. An O2 dimer distant from B causes only weak carrier recombination, which nevertheless drives O2 diffusion under light to form the BO2 complex. Although BO2 and O2 produce nearly identical defect levels in the band gap, the recombination at BO2 is substantially faster than at O2 because the charge state of the latter inhibits the hole capture step of recombination.