Opioid Use Disorder (OUD) is a complex, chronic condition affecting nearly 70,000 Veterans who can experience significantly reduced quality of life (e.g., poorer social, occupational, and health-related functioning). VA clinics providing Medication treatment for OUD (MOUD; e.g., buprenorphine, methadone), the 1st-line treatment for OUD, often face challenges in also attempting to treat Veteran functional needs, which may require them to extend beyond their available resources to provide support. There is an urgent need for functionally impactful and accessible treatments for Veterans in MOUD. Acceptance and Commitment Therapy (ACT) is a well-suited framework to support the functioning of Veterans in MOUD with over 20 years of research support. However, the traditional practice ACT requires a trained clinician to provide weekly, hour-long therapy sessions (typically for 12-16 weeks) and may be too burdensome for MOUD clinics to use alongside standard care. Fortunately, emerging research suggests that mobile health interventions (MHIs; e.g., smartphone apps) can overcome many of these pragmatic barriers. MHIs can efficiently deliver functionally-focused treatments focused on Veteran functioning in real-world settings, through minimally burdensome and accessible formats. Currently however, no MHI's targeting functioning exist for Veterans in MOUD. The proposed study will address this gap by developing and evaluating an early prototype of a targeted smartphone app designed to enhance the functional outcomes of Veterans receiving MOUD called ACT to RECOVER (Acceptance and Commitment Therapy to Reach Empowerment through Commitment, Openness, and Valuing Experiences in Recovery). The study will occur in 3 phases: Phase 1: Development. Develop content for ACT to RECOVER using Veteran (n=10) and provider feedback (n=10). Phase 2: Iterative Usability Assessment. Conduct field testing (3 rounds, n=4-5 per round) to refine ACT to RECOVER format, acceptability, and usability. Phase 3: Pilot ACT to RECOVER in a Stage 1b Randomized Controlled Trial (RCT). Conduct a pilot trial to compare ACT to RECOVER (n=20) to a smartphone-based symptom monitoring control group (n=20). * (3a) Evaluate the acceptability and feasibility of each condition's app and study procedures. * (3b) Explore changes in functional (e.g., values-based living, quality of life) and clinical outcomes (e.g., illicit opioid use) which will be key outcomes in future efficacy testing.