The investigators propose to monitor diurnal fluctuations in IOP and choroidal blood flow (Pulsatile Ocular Blood Flow,POBF), and intervisit ONH topographical and blood flow changes—ie to monitor the direct ONH consequences of IOP . Open angle glaucoma patients are commonly prescribed topical latanoprost as first line therapy. The EXACCT study, for which I was the principal investigator and which is now submitted for publication, demonstrated that COSOPT was an efficacious choice as second line therapy for patients not controlled on latanoprost monotherapy. The investigators will therefore recruit 20 OAG patients on latanoprost monotherapy, perform diurnal curves of IOP, as well as a.m. ONH morphology and ONH blood flow. Cosopt will then be added and at the next visit the same measurements will be repeated.