<prompt> The most common organism causing osteomyelitis isE, coliPseudomonasKlebsiellaStaph aureus  <response>: The most common organism in osteomyelitis in intravenous drug users is staphylococcus aureus. Pseudomonas aeruoginosa an otherwise unusual cause of osteomyelitis, is the second most frequently observed microorganism Refer Maheshwari 6the p 168 