Inclusion Criteria:  Adult OAB patients ≥20 years Diagnosed with moderate to severe OAB (with or without urgency incontinence) based on OABSS >5 and clinical assessment, with UUI-predominant presentation, for at least 3 months Able to receive TTNS and accommodate treatment logistics (30 min per on-site session, twice weekly) Provided informed consent to participate in the study  Exclusion Criteria:  Neurologic conditions associated with OAB symptoms History of stress urinary incontinence Use of intravesical onabotulinumoxinA within recent 6 months Postvoid residual urine volume (PVR) ≥ 100mL Evidence of active urinary tract infection or urinary tract stone at screening Genitourinary tract operation during the 3-month period prior to baseline Confirmed or suspected genitourinary tract or pelvic malignancy History of uncontrolled hypertension (systolic >160 mmHg and/or diastolic >110 mmHg) History of intolerance to mirabegron Patients with pacemakers or implantable defibrillators Patients prone to excessive bleeding Patients with nerve damage that could impact percutaneous tibial nerve or pelvic floor function Patients who are pregnant or planning to become pregnant during the duration of treatment History of medical conditions or presence of patient factors that, in the judgement of the investigator, would preclude adherence to study protocol