  Boron suboxide B6O, the hardest known oxide, has an R-3m crystal structure
({\alpha}-B6O) that can be described as an oxygen-intercalated structure of
{\alpha}-boron, or, equivalently, as a cubic close packing of B12 icosahedra
with two oxygen atoms occupying all octahedral voids in it. Here we show a new
ground state of this compound at ambient conditions, Cmcm-B6O (\b{eta}-B6O),
which in all quantum-mechanical treatments that we tested (GGA, LDA, and hybrid
functional HSE06) comes out to be slightly but consistently more stable.
Increasing pressure and temperature further stabilize it with respect to the
known {\alpha}-B6O structure. \b{eta}-B6O also has a slightly higher hardness
and may be synthesized using different experimental protocols. We suggest that
\b{eta}-B6O is present in mixture with {\alpha}-B6O, and its presence accounts
for previously unexplained bands in the experimental Raman spectrum.
