OAG Board of Directors
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AnesthesiologistMurtaza Diwan, MD
Chair -
AnesthesiologistAri Tallen, MD
Chair Elect -
AnesthesiologistMichelle Smith, MD
Secretary -
AnesthesiologistNikki Jaworski, MD
Treasurer -
AnesthesiologistLauren Schergen, MD
Board Member -
AnesthesiologistMelissa Welker, MD, MPH
Board Member -
AnesthesiologistAndy Neice, MD
Ex-Officio Board Member
Clinical Leaders
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Anesthesiologist
Andy Neice, MD
Chief Medical Officer -
Anesthesiologist
Ari Tallen, MD
Critical Care Medical Director -
AnesthesiologistRahul Raghavan, MD
Clinical Quality Director -
AnesthesiologistAbbie Tirrell, MD, MPH
Physician Billing Compliance Officer -
AnesthesiologistOmar Halawa, MD
ASC Medical Director -
Advanced Practice ProviderDominique Irigoin, CRNA
Chief APP
Corporate Leadership
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Lorissa Addabbo
Chief Executive OfficerAs Chief Executive Officer, Lorissa oversees the business, administrative and financial operations of the corporation. Her staff addresses compliance, contracting with hospitals and ambulatory surgery centers, insurance reimbursement negotiation, physician & CRNA recruitment, scheduling, credentialing and information technology.
Lorissa brings a wealth of expertise and leadership to OAG. Most recently, she served as Vice President of Anesthesia, Surgical Services, and Neuromuscular Service Lines at Salem Health, which informs her first-hand knowledge of OAG’s value and quality. During her tenure she realized that she prefers to work for independent practices, as she had done for seven years as the CEO of Hope Orthopedics. Her earlier leadership roles at Adventist Medical Center, Aldrich Advisors, and Salem Health round-out her extensive knowledge of the Pacific Northwest healthcare market.
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Meredith Johnson
Chief Financial OfficerMeredith Johnson is an accomplished financial executive serving as Chief Financial Officer of Oregon Anesthesiology Group, where she oversees the organization’s financial strategy, operations, and long-term fiscal health. Meredith works closely with physicians and administrative teams to optimize performance, support sustainable growth, and ensure high-quality patient care remains at the forefront of the organization’s mission. With extensive experience in healthcare finance, Meredith has held CFO and senior financial leadership roles with physician groups, ambulatory surgery centers, healthcare organizations, and insurance companies, with expertise spanning financial planning, physician compensation, revenue cycle management, strategic growth, and operational improvement. She holds a Bachelor of Arts in Accounting from the University of Oregon and is a Certified Public Accountant licensed in Oregon.
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Trang Nguyen
Director of OperationsTrang has been with OAG for nearly 20 years. As Director of Operations, Trang is an integral member of the Executive Leadership Team and provides a high-level of support to the Executive Leadership Team and Board of Directors at OAG. Her responsibilities include overseeing the staffing efficiencies of OAG’s clinical workforce for optimal practice management, and various other projects that support an integrated approach for organization-wide improvements.
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Micheal McCarthy
Human Resources DirectorMicheal joined OAG in 2016 as Human Resources Director. In his role, he is responsible for all recruitment activities, human resources and credentialing, employee relations, patient satisfaction surveys, performance reviews and leadership development.
Micheal has a Bachelor of Arts in Management and Organizational Leadership from George Fox University, and a Master of Science in Counseling Psychology from Lewis and Clark. He has more than 20 years of experience in Human Resources working with companies in retail, technology, healthcare, and most recently an international design firm.
Micheal and his wife have 2 great kids, and a feisty jack russell terrier. He enjoys the outdoors of the Pacific Northwest and is an avid trail runner.
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Siân Williams
Associate Director, Revenue Strategy and OperationsSiân overseas all area of payor contract management. She has over 20 years of experience in healthcare. She received her Bachelors from University of Oregon.
Siân joined OAG in 1993 and brings 14 years of experience managing payor contracting, negotiations, and analysis. Sian serves as billing liaison to our billing vendor, serves on various internal billing committees and is a non-voting member of Legacy Health Partner’s Finance and Contracting Committee. Prior to becoming OAG's Payer Contract Manager, Siân served in various roles ranging from Contracts Analyst, Contracts Administrator and Quality Assurance Administrator, overseeing risk management, credentialing and contract coordination.
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Cate Gomez
Compliance and Privacy OfficerCate joined OAG in 2015 and is principally responsible for managing the OAG compliance program. The program helps ensure that we have the structure and processes in place to conduct business ethically and in accordance with the law.
Key program elements include OAG policies, training and communications, routine auditing and monitoring, as well as high-level oversight. Cate is also involved in preparing and negotiating contracts for OAG, as well as providing consultation and leadership on a variety of business issues.
