Can oxybutinin be used to treat non bacterial prostatitis/interstitial cystitis/chronic pelvic pain syndrome?

Short no: It is not useful as symptom complex is much different then that for which oxybutinin is used for. You need evaluation and then specific rx.

Yes: Oxybutynin is an antimuscarinic and primarily used for the symptoms of urinary frequency, urgency, and urge incontinence. Though most commonly used as part of the treatment of overactive bladder and neurogenic bladder-related detrusor instability it can be used for any condition or symptom complex with bothersome bladder-related urinary urgency and frequency. It treats the symptom, not the disease.