In this study we will determine whether CPAP applied at night changes IOP and ocular perfusion pressure (OPP). We will also assess its possible impact on ocular microvasculature. Two groups of patients will be included: those with POAG and OSA, and those with OSA without glaucoma. They will attend for two overnight assessments: the first before starting CPAP and the second 4-6 weeks into the treatment. Repeated measurements of IOP at night will be performed and participants will continue self-measuring IOP at home in the day. An Ocular Coherence Tomography Angiography (OCT Angiography) of the optic disc and the surrounding retina will be performed at baseline and after a few weeks of CPAP treatment.