  We have carried out a search for 18-cm OH megamaser (OHM) emission with the
Green Bank Telescope. The targeted galaxies comprise a sample of 121 ULIRGs at
0.09<z<1.5, making this the first large, systematic search for OHMs at z>0.25.
Nine new detections of OHMs are reported, all at redshifts z<0.25. For the
remainder of the galaxies, observations constrain the upper limit on OH
emission; this rules out OHMs of moderate brightness (L_OH > 10^3 L_sun) for
26% of the sample, and extremely bright OHM emission (L_OH > 10^4 L_sun) for
73% of the sample. Losses from RFI result in the OHM detection fraction being
significantly lower than expected for galaxies with L_IR >10^12 L_sun. The new
OHM detections are used to calculate an updated OH luminosity function, with
\Phi[L]\simL_OH^{-0.66}; this slope is in agreement with previous results.
Non-detections of OHMs in the COSMOS field constrain the predicted sky density
of OHMs; the results are consistent with a galaxy merger rate evolving as
(1+z)^m, where m<6.
