Normobaric Oxygen Therapy in Acute Ischemic Stroke Trial

Inclusion Criteria: * Age greater than or equal to 18 years. * Acute ischemic stroke in whom treatment can potentially be started within 9 hours after symptom onset. If the symptom onset time is unknown, the time of onset will be defined as the midpoint between the time when the subject was last seen neurologically intact, and when found to have a neurological deficit. * National Institutes of Health Stroke Scale (NIHSS) score 4 or greater. Exclusion Criteria: * Patients being actively considered for intravenous or intra-arterial thrombolysis will be excluded. * Patients likely to have acute stroke intervention such as carotid endarterectomy or stent or angioplasty, hemicraniectomy, etc. * Rapidly improving neurological deficits (transient ischemic attack). * Known history of severe chronic obstructive pulmonary disease (Forced Expiratory Vital Capacity less than 1.0 or oxygen dependent). * More than 3 L/min oxygen required to maintain peripheral arterial oxygen saturation above 92%. * New York Heart Association Class III heart failure. * Endotracheal intubation prior to enrollment or impending need for artificial ventilation. * Coma (National Institutes of Health Stroke Scale item 1a score of 3). * Suspected seizure at or after onset of stroke, or a known active seizure disorder. * Blood glucose below 50 mg/dL or greater than 250 mg/dL prior to enrollment. * Concurrent severe non-stroke medical illness requiring admission to a non-neurological intensive care unit * Expected survival less than 90 days. * Any condition that might limit neurological assessment or follow-up in the opinion of the investigator. * Pre-menopausal women with a positive pregnancy blood test performed at admission. * Inability to obtain consent from the patient or legally authorized representative. * Active participation in another intervention study (e.g. investigational drug trial). * Proven alternate etiology for stroke-like symptoms.