  We present mid-infrared observations for a sample of 16 O-type stars. The
data were acquired with the NASA Spitzer Space Telescope, using the IRS
instrument at moderate resolution (R $\sim$ 600), covering the range $\sim
10-37$ microns. Our sample includes early, mid and late O supergiants and
dwarfs. We explore for the first time their mid-IR spectral morphology in a
quantitative way. We use NLTE expanding atmosphere models to help with line
identifications, analyze profile contributions and line-formation regions. The
O supergiants present a rich emission line spectra. The most intense features
are from hydrogen - $6\alpha$, 7$\alpha$, and $8\alpha$ - which have
non-negligible contributions of HeI or HeII lines, depending on the spectral
type. The spectrum of early O supergiants is a composite of HI and HeII lines,
HeI lines being absent. On the other hand, late O supergiants present features
composed mainly by HI and HeI lines. All emission lines are formed throughout
the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum.
Two stars of our sample exhibit very similar mid-IR features, despite having a
very different optical spectral classification. The analysis of O-type stars
based on mid-IR spectra alone to infer spectral classes or to estimate physical
parameters may thus be prone to substantial errors. Our results may therefore
inform spectroscopic observations of massive stars located in heavily obscured
regions and help establish an initial framework for observations of massive
stars using the Mid-Infrared Instrument on the James Webb Space Telescope.
