I Am Taking Blood Pressure Medications, That Include Lozide And

I am taking blood pressure medications, that include Lozide and Diovan, as well as micro K 16 mEq in the morning and 8 mEq in the afternoon. My doctor prescribed oxybutynin for overactive bladder. The pharmacist warns me about using the 2 drugs together. I asked her about the timing, she said wouldn't make any difference. I would like to know if I take the bladder medication late in the day, perhaps even just before bed, if that would help. I am not really clear on the problems associated with using the two drugs. Can you help clear this up for me?