We present mid-infrared ( 5 - 40 \micron ) spectra of shocked ejecta in the Galactic oxygen-rich supernova remnant G292.0+1.8 , acquired with the IRS spectrograph on board the Spitzer Space Telescope . The observations targeted two positions within the brightest oxygen-rich feature in G292.0+1.8 . Emission lines of [ Ne II ] \lambda 12.8 , [ Ne III ] \lambda \lambda 15.5,36.0 , [ Ne V ] \lambda 24.3 and [ O IV ] \lambda 25.9 \micron are detected from the shocked ejecta . In marked contrast to what is observed in Cassiopeia A , no discernible mid-IR emission from heavier species such as Mg , Si , S , Ar or Fe is detected in G292.0+1.8 . We also detect a broad emission bump between 15 and 28 \micron in spectra of the radiatively shocked O-rich ejecta in G292.0+1.8 . We suggest that this feature arises from either shock-heated Mg _ { 2 } SiO _ { 4 } ( forsterite ) dust in the radiatively shocked O-rich ejecta , or collisional excitation of PAHs in the blast wave of the SNR . If the former interpretation is correct , this would be the first mid-IR detection of ejecta dust in G292.0+1.8 . A featureless dust continuum is also detected from non-radiative shocks in the circumstellar medium around G292.0+1.8 . The mid-IR continuum from these structures , which lack mid-IR line emission , is seen in Chandra images as bright X-ray filaments , is well described by a two-component silicate dust model . The temperature of the hot dust component ( M _ { d } \sim 2 \times 10 ^ { -3 } M _ { \odot } ) is \sim 115 K , while that of the cold component ( roughly constrained to be \lesssim 3 M _ { \odot } ) is \sim 35 K. We attribute the hot component to collisionally heated dust in the circumstellar shocks in G292.0+1.8 , and attribute the cold component to dust heated by the hard FUV radiation from the circumstellar shocks . Using average O/Ne and O/Si mass ratios measured for a sample of ejecta knots in the X-rays , our models yield line strengths consistent with mass ratios { M _ { O } / M _ { Ne } } \approx 3 , { M _ { O } / M _ { Si } } \gtrsim 61 and { M _ { O } / M _ { S } } \approx 50 . These ratios ( especially the large O/Ne mass ratio ) are difficult to reproduce with standard nucleosynthesis models of well-mixed supernova ejecta . This reinforces the conclusions of existing X-ray studies that the reverse shock in G292.0+1.8 is currently propagating into the hydrostatic nucleosynthetic layers of the progenitor star , and has not yet penetrated the layers dominated by explosive nucleosynthetic products .