Overview of OH ²Π_1∕2 (J = 3∕2−1∕2) transition towards the sample. The blue dotted line marks the systemic LSR velocity of the source in the signal band. The positions of the hfs lines are labelled in red in the signal and in green in the image band. The HFS fit to the spectra is shown in light blue. The velocity resolution is 0.75 km s⁻¹ for all sources, except for G351.58−0.4, where we show the spectrum at a velocity resolution of 1.5 km s⁻¹.