i have a lot of trouble peeing while on opiates, so usually now i just ignore the urge and it goes away, and then once i really have to i just sit down (i'm a girl) and clear my mind and relax, and it'll come.
especially with high doses of oxy, it's like i have to keep pushing it out even after the flow has started. it doesn't bother me too much though because it doesn't cause me that much discomfort.