Trauma-related injuries: The patient's history of a fall and straining to put an O2 bottle in a chair holder suggests that the most likely cause of the pain is a direct result of these incidents, possibly leading to bruises, strains, or minor fractures.

Muscle strain: Overexertion from trying to maneuver the O2 bottle could have strained the muscles in the buttocks and knee, leading to pain.
Contusion: The fall could have resulted in a contusion (bruise) in the buttocks area, causing pain and discomfort.
Ligament sprain: The knee pain could be due to a sprained ligament from the sudden movement or twist while trying to put the O2 bottle in the chair holder.

Fracture: Although less likely, it's crucial not to miss a potential fracture in the buttocks or knee area, especially given the mechanism of injury (fall and straining). A fracture would require immediate medical attention.
Deep vein thrombosis (DVT): Given the patient's need for an O2 bottle, there might be underlying health conditions that increase the risk of DVT, which could cause knee pain and is a potentially life-threatening condition if not treated promptly.
Infection or abscess: If the skin was broken during the fall, there's a risk of infection, which could lead to more severe complications if not addressed.

Avulsion fracture: A rare possibility where a fragment of bone is pulled away from the main mass of bone due to physical trauma, which could be a result of the fall or straining.
Bursitis: Inflammation of the bursae (fluid-filled sacs that cushion bones, tendons, and muscles near joints) could cause pain in the knee or buttocks area, though this would be less directly related to the described incidents.
Neurological conditions: Conditions affecting the nerves, such as neuropathy or radiculopathy, could cause pain in the buttocks and knee but would be less likely given the acute onset related to specific incidents.