  In strongly correlated multi-orbital systems, various ordered phases appear.
In particular, the orbital order in iron-based superconductors attracts much
attention since it is considered to be the origin of the nematic state. In
order to clarify the essential condition for realizing orbital orders, we study
simple two-orbital ($d_{xz}$, $d_{yz}$) Hubbard model. We find that the orbital
order, which corresponds to the nematic order, appears due to the vertex
corrections even in the two-orbital model. Thus, $d_{xy}$ orbital is not
essential to realize the nematic orbital order. The obtained orbital order
depends on the orbital dependence and the topology of fermi surfaces. We also
find that another type of orbital order, which is rotated $45^\circ$, appears
in the heavily hole-doped case.
