The researchers want to investigate how fluid intake influences the symptoms of OAB. The researchers will recruit 45 patients with OAB and randomize them (blind) in 3 groups (low/normal/high fluid intake). Patients will follow this drink pattern for 2-5 days, while filling in an standard micturation diary for 2 days. This micturation diary is coupled to: collection of multiple urine samples, urine pH measurements, and a Perception of Intensity of Urgency Scale (PPIUS) scoring table. All this is collected by the patients themselves. Urine samples will be analysed for osmolality by the clinical chemistry department.