I find that sleeping semi-upright greatly improves my osa. will a cpap machine do more for me?

Most likely: You need to have a polysomnogram study by a MD who specializes in Sleep Medicine. That will diagnose your problem and the best solution.

CPAP is mainstay ...: OSA is a condition of intermittent upper airway collapse during sleep causing disordered breathing, disturbed sleep and daytime sleepiness. Diagnosis is confirmed by Sleep Specialist and Sleep Study (Polysomnograph). Weight loss, smoking cessation, alcohol reduction are important. Gold Standard Treatment is with CPAP machine. Altering postion (preventing one lying flat on one's back) can help.