  Using ab initio calculations, we have investigated an insulating tetragonally
distorted perovskite BaCrO$_3$ with a formal $3d^2$ configuration, the volume
of which is apparently substantially enhanced by a strain due to SrTiO$_3$
substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects
leads to a metal-insulator transition and in-plane zigzag orbital-ordering (OO)
of alternating singly filled $d_{xz}+id_{yz}$ and $d_{xz}-id_{yz}$ orbitals,
which results in a large orbital moment $M_L$ ~ -0.78 $\mu_B$ antialigned to
the spin moment $M_S$ ~ $2|M_L|$ in Cr ions. Remarkably, this ordering also
induces a considerable $M_L$ for apical oxygens. Our findings show
metal-insulator and OO transitions, driven by an interplay among strain,
correlation, and SOC, which is uncommon in 3d systems.
