Results of observations of circumstellar OH masers in lines with wavelengths near 18 cm are reported. The observations were carried out on the radio telescope of the Nan cay Radio Astronomy Observatory (France). In 2007-2009, 70 late-type stars were observed (including Mira and semi-regular variables). For 53 of these, emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Polarized emission features were detected in the OH line spectra of 41 stars. A summary of all the observations is given. The results obtained for T Lep, R LMi, and R Crt are discussed. Emission in the 1665 and 1667 MHz OH lines was detected in T Lep for the first time. Features probably due to Zeeman splitting were detected in the OH line profiles of all three stars. Estimates of the magnetic-field strengths in the maser sources were obtained (0.46-2.32 mG). Variability of the polarization characteristics of the maser emission of the stars on time intervals of several months was found.