  The effect of deposition oxygen pressure (P$_{O}$) on phase separation (PS)
induced in epitaxial La$_{0.67}$Ca$_{0.33}$MnO$_{3}$/NdGaO$_{3}$(001) films was
investigated. Fully oxygenated films grown at high P$_{O}$ are anisotropically
strained. They exhibit PS over a wide temperature range, because of the large
orthorhombicity of NdGaO$_{3}$ substrates. The paramagnetic
insulator-to-ferromagnetic metal (FM) and FM-to-antiferromagnetic insulator
(AFI) transitions gradually shift to lower temperatures with decreasing PO. The
AFI state is initially weakened (P$_{O}$ >= 30 Pa), but then becomes more
robust against the magnetic field (P$_{O}$ < 30 Pa). The out-of-plane film
lattice parameter increases with decreasing P$_{O}$. For films grown at
P$_{O}$>= 30 Pa, the slight oxygen deficiency may enlarge the lattice unit
cell, reduce the anisotropic strain and suppress the AFI state. Films deposited
at P$_{O}$ < 30 Pa instead experience an average compressive strain. The
enhanced compressive strain and structural defects in the films may lead to the
robust AFI state. These results aid our understanding of PS in manganite films.
