  We attribute the structural phase transition (SPT) in the parent compounds of
the iron pnictides to orbital ordering. Due to the anisotropy of the $d_{xz}$
and $d_{yz}$ orbitals in the $xy$ plane, a ferro-orbital ordering makes the
orthorhombic structure more energetically favorable, thus inducing the SPT. In
this orbital-ordered system, the sites with orbitals that do not order have
higher energies. Scattering of the itinerant electrons by these localized
two-level systems causes a resistivity anomaly upon the onset of the SPT. The
proposed orbital ordering also leads to the stripe-like anti-ferromagnetism and
anisotropy of the magnetic exchanges. This model is quantitatively consistent
with available experimental observations.
