  We study a spin $S$ quantum Heisenberg model on the Fe lattice of the
rare-earth oxypnictide superconductors. Using both large $S$ and large $N$
methods, we show that this model exhibits a sequence of two phase transitions:
from a high temperature symmetric phase to a narrow region of intermediate
``nematic'' phase, and then to a low temperature spin ordered phase.
Identifying phases by their broken symmetries, these phases correspond
precisely to the sequence of structural (tetragonal to monoclinic) and magnetic
transitions that have been recently revealed in neutron scattering studies of
LaOFeAs. The structural transition can thus be identified with the existence of
incipient (``fluctuating'') magnetic order.
