We present the results of a long-term near infrared monitoring program (observations in the J, H and K bands) which was carried out on a selected sample of 25 OH/IR stars included in the IRAS Point Source Catalogue. They show a wide variety of infrared and OH maser luminosities, expansion velocities and LRS types. In the IRAS two-colour diagram [12]-[25] vs. [25]-[60] they are also located at different positions along the sequence of colours typically associated with OH/IR stars. The observations have been carried out using the infrared 1.5 m Sanchez Magro Telescope (SMT) at Izana (Tenerife, Spain) since the beginning of 1991 until the end of 1997. We have derived periods and amplitudes for fourteen stars, obtaining their light curves, finding in some cases that the light curves are asymmetric and changes in the mean magnitudes with time. Furthermore, we have looked for the optical counterpart of these stars, identifying fifteen of them, whereas nine did not present any optical counterpart.