An oxygen-sensitive mode in oxygenated wild-type cytochrome P450cam (oxyP450cam) is observed at 401 cm(-1) and assigned to the delta(Fe-O-O) bending mode, based upon O-16(2),O-18(2) isotopic shifts (19 cm(-1)) and comparison with Co- and Fe-oxyporphyrin complexes. The detection of this Fe-O-O bending mode has structural implications for enzyme function since its frequency reflects the energies associated with Fe-O-O distortion in oxyhemeprotein active sites. Three body normal coordinate calculations adequately fit the experimental data set with a 125-130 degrees bond angle for the Fe-O-O linkage in oxyP450cam. Observation of low-frequency isotope-sensitive vibrational patterns, some of which an hypothesized to be associated with out-of-plane porphyrin motions are also reported. These patterns, in conjunction with the high frequency of this bending mode and the abnormal isotopic shift of the Fe-O-2 stretching mode, suggest a "strained" Fe-O-O moiety in oxyP450cam, with comparable mobility to HbO(2) and MbO(2). Possible sources of this "strain" and implications for catalytic dioxygen activation in P450cam an discussed.