Vestibular dysfunction: Given the patient's recent COVID-19 illness, vestibular dysfunction is a plausible cause of dizziness. COVID-19 has been associated with various neurological manifestations, including vestibular symptoms.

Postural orthostatic tachycardia syndrome (POTS) variant: Although orthostatic BP readings are fine, some patients with POTS may not always exhibit significant orthostatic hypotension. The recent COVID-19 illness could be a triggering factor.
Anxiety or depression: These conditions are common in elderly patients and can cause dizziness. The patient's recent illness and ongoing oxygen therapy could contribute to anxiety or depression.
Medication side effects: Although not specified, the patient may be taking medications that could cause dizziness as a side effect.

Cardiac arrhythmia: Despite a normal Holter event monitor and EKG showing sinus brady (which has been stable), it is essential to consider the possibility of intermittent arrhythmias that may not have been captured during monitoring.
Cerebrovascular disease: A stroke or transient ischemic attack (TIA) could cause dizziness, and it is crucial to rule out these conditions, especially in an elderly patient.
Intracranial hypotension: This condition can cause dizziness and has been associated with recent illnesses, including COVID-19.

Autoimmune inner ear disease: This is a rare condition that can cause vestibular symptoms, including dizziness.
Superior canal dehiscence syndrome: A rare condition that affects the inner ear and can cause dizziness, especially with changes in pressure.
Multiple system atrophy (MSA): A rare neurodegenerative disorder that can cause autonomic dysfunction, including orthostatic hypotension and dizziness.