The Ozone Monitoring Instrument (OMI) is a nadir viewing wide field imaging spectrometer for ozone monitoring. The instrument is the Dutch/Finnish contribution to the NASA EOS-AURA mission. OMI observes earth's back scattered radiation in two spectral channels: the UV channel (270 nm - 350 nm) and the VIS channel (350 nm - 500 nm). Each channel employs a CCD detector (576 X 780 px). The extreme wide field of view of 114 degrees, equal to a swath wide of 2600 km, is obtained by an all reflective telecentric telescope and enables global ozone coverage in one day. Other key features are the spectral range (270 nm - 500 nm) and resolution (spectral sampling distance 0.15 - 0.32 nm/px), the application of a polarization scrambler and its compact design (400 X 300 X 500 mm). Excellent stray light performance in the UV channel is obtained by an elegant opto-mechanical design of the UV optics where the UV wavelength range is split in two parts with separate optical paths and the separate spectra are imaged on one CCD. Onboard calibration includes a white light source, LEDs, and multi-surface solar-calibration diffuser. The OMI-EOS project follows a Proto-Flight approach, supported by breadboards and engineering qualification models on parts and sub-system level. In order to increase confidence in the design, the instrument development model was built. During intensive testing critical performance parameters were checked , e.g. UV stray light behavior, polarization sensitivity, distortion, spatial and spectral ranges and resolutions.