The output mode cleaner (OMC) cavity cleans the beam transmitted at the anti-symmetric interferometer port of all higher-order modes and RF sidebands (introduced for the interferometer control, see Sect. 2.4.2 ). The replacement of the two low finesse output mode cleaner (OMC) cavities with a high finesse one was one of the major upgrades between the O3 and O4 observation runs. The main motivation for this upgrade is to reduce losses at the output port of the interferometer in order to reduce the quantum noise. In Table 10.1 the losses budget measured during the observing runs O3 and expected for O4 for the detection system is detailed. The total losses should be reduced by a factor 2.