Herschel-HIFI observation of four extreme OH/IR stars. The spectra have been corrected for the LSR velocity for each object and rebinned to ~2 km s^-1 sampling. The vertical lines indicate the expected position of the HO, HO, and HO 3₁₂−3₀₃ transitions, from left to right. The expected position of the HO 1₁₁−0₀₀ transition (dashed line) from the upper side-band can be seen at the far right.