The IRAS infrared colors, (60 - 25)μm and (25 - 12)μm, allow efficient identification of Type II OE/IR stars. We present Arecibo1 OH (1612 MHz) observations of 474 IRAS point sources chosen to define the exact regions of the two-color diagram occupied by OH/IR stars. Our observations are complete within the boundary regions of the two-color locus and within the region, 16h right ascension < 22h, 0° < declination < 37°. The sensitivity of the Arecibo telescope allows the identification of many weak sources that would not have been detected by previous surveys and the weak end of the masing phenomenon has been studied for the first time.