These observations aim to directly investigate, for the first time, the operation of the proposed IR radiative pump mechanism for the OH maser emission observed from cool AGB and supergiant stars. The proposed OH maser pump scheme involves absorption in the 34.6um and 53.3um doublets from the OH 2Pi(3/2) J = 3/2 ground rotational state state, to excited 2Pi(1/2) rotational states, followed by a cascade network back to the ground state, involving the emission of photons at 98.7, 163.2 and 79.1um; and at 48.7, 96.3 and 119.2um, leading to inversion of the ground state hyperfine state populations and maser emission. The 34.6 and 53.3um pump lines will be observed with the SWS and LWS Fabry-Perots (FPs), respectively, while all the predicted cascade lines will be observed with the LWS FPs. The proposal requests time to supplement observations begun in the LWS Consortium's GT programme MBARLOW.HIRES. Originally, the OH lines from 15 stars were to be observed with the LWS and SWS FPs. However, the decreased in-flight sensitivities of the LWS and SWS, due to the higher than expected particle hit-rates, led to a large increase in the required exposure times. As a result only two sources (the F8Ia supergiant IRC+10420 and the archetype OH/IR star WX Psc = IRC+10011) can now be observed within the 14 hours available in the LWS HIRES programme. We request sufficient time to add four more bright archetypal OH maser sources to the sample, namely the mainline (Type I) OH maser source W Hya (a semi-regular variable), the mixed 1612 MHz and mainline (Type I/II) maser source IK Tau (a Mira variable), the very red OH/IR source OH26.5+0.6 (a very strong 1612 MHz, or Type II, OH maser source) and NML Cyg, an extremely luminous non-variable M supergiant Type II OH maser source. Our GT OH observations of WX Psc unfortunately have a relatively low scheduling priority (Grade 4). Since the required observation time is lengthy, they thus have a low probability of getting scheduled. We therefore request an upgrade of the Priority of this source to 1 (=Grade 8).