Using the SWS and LWS instruments on board of the ISO spacecraft, we searched for OH-lines in the FIR spectra of the two OH megamaser galaxies IRAS 20100-4156 and 3 Zw 35. The lines at 34µ and 53µ were detected in absorption with certainty in the spectrum of IRAS 20100-4156 and probably in the spectrum of 3 Zw 35. From the measured equivalent widths we derived column densitites of NOH≃(2.2±0.4)x1017cm–2 and NOH≃(2.0±0.6)x1017cm–2, respectively. The 119µ line was tentatively detected in the spectrum of IRAS 20100-4156, but definitely is much weaker than would be expected from the strengths of the lines at 34µ and 53µ. In the spectrum of 3 Zw 35 the 119µ line was not found. We take this as an indication that at 119µ the continuum source is considerably larger than at 34µ and 53µ and is covered only partly by the absorbing OH cloud. - The finding that the OH column density is similar in both objects despite substantial differences in their IR and maser luminosities, is taken as support of the hypothesis that OH megamasers are radiatively pumped by the 34µ and/or the 53µ lines.