We present Spitzer IRS observations of a small sample of heavily obscured IRAS sources displaying both the infrared and OH maser emission characteristic of OH/IR stars on the asymptotic giant branch (AGB), but also radio continuum emission typical of ionized planetary nebulae (PNe), the so-called OHPNe. Our observations show that their mid-infrared spectra are dominated by the simultaneous presence of strong and broad amorphous silicate absorption features together with crystalline silicate features, originated in their O-rich circumstellar shells. Of the five sources observed, three are clearly nonvariable at infrared wavelengths, confirming their post-AGB status, while the remaining two still show strong photometric fluctuations and may still have not yet departed from the AGB. One of the nonvariable sources in the sample, IRAS 17393-2727, displays a strong [Ne II] nebular emission at 12.8 μm, indicating that the ionization of its central region has already started. This suggests a rapid evolution from the AGB to the PN stage. We propose that these heavily obscured OHPNe represent the population of high-mass precursors of PNe in our Galaxy.