Overactive bladder is a condition that is characterized by sudden, involuntary contraction of the muscle in the wall of the urinary bladder. This results in a sudden and unstoppable need to urinate, even though the bladder may only contain a small amount of urine. The urge may be difficult to stop, and overactive bladder may lead to urinary frequency and incontinence. Overactive bladder (OAB) is a common condition that affects millions of Americans. Having to go to the bathroom many times during the day and night (nocturia) is another symptom of OAB.