  Isotropic odd unitary groups generalize Chevalley groups of classical types
over commutative rings and their twisted forms. Such groups have root subgroups
parameterized by a root system $\mathsf{BC}_\ell$ and may be constructed by
so-called odd form rings with Peirce decompositions. We show the converse: if a
group $G$ has root subgroups indexed by roots of $\mathsf{BC}_\ell$ and
satisfying natural conditions, then there is a homomorphism $\mathrm{StU}(R,
\Delta) \to G$ inducing isomorphisms on the root subgroups, where
$\mathrm{StU}(R, \Delta)$ is the odd unitary Steinberg group constructed by an
odd form ring $(R, \Delta)$ with a Peirce decomposition. For groups with root
subgroups indexed by $\mathsf A_\ell$ (the already known case) the resulting
odd form ring is essentially a generalized matrix ring.
