  A major obstacle to using SQUIDs as qubits is flux noise. We propose that the
heretofore mysterious spins producing flux noise could be $O_2$ molecules
adsorbed on the surface. Using density functional theory calculations, we find
that an $O_2$ molecule adsorbed on an {\alpha}-alumina surface has a magnetic
moment of ~1.8 {\mu}B. When the spin is oriented perpendicular to the axis of
the O-O bond, the barrier to spin rotations is about 10 mK. Monte Carlo
simulations of ferromagnetically coupled, anisotropic XY spins on a square
lattice find 1/f magnetization noise, consistent with flux noise in Al SQUIDs.
