OA is an abbreviation of 'osteoarthritis'.

The joint surfaces within the knee showing osteoarthritis breakdown of the usually shiny white joint cartilage - you can even see the yellower bone showing underneath.
Synonyms:
osteoarthritis
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FORUM DISCUSSIONS
What's the difference OA / Chondromalacia ?
CLINICAL PUBLICATIONS
Towards secondary prevention of early knee osteoarthritis. Mahmoudian A, Van Assche D, Herzog W and Luyten FP. RMD Open. 2018; 4(2): e000468.