Well, swelling of the ‘thumb joint’ does not necessarily mean osteoarthritis, especially if it is just a single joint involved. Other possible causes include cysts, infections, inflammations, soft tissue growths, bony or cartilage outgrowths, bunions etc. It would be best to get this evaluated by an orthopedician/ rheumatologist, for an accurate diagnosis and appropriate management. If indeed it turns out to be osteoarthritis, increased movements are best avoided around the joint, especially those that involve rapid repetitive motion (such as in piano playing), while graded smooth movements may help with the stiffness, if any. Still, it would be best to let your doctor answer the question after the evaluation.
Hope this was useful.