This is a National Institute for Health Research (NIHR) Efficacy and Mechanism Evaluation (EME) clinical trial aiming to investigate a possible new treatment to limit damage to the brain caused by a stroke. Strokes that are caused by a clot blocking an artery in the brain ('ischaemic' strokes) starve brain tissue of oxygen and nutrients. Over a short period of time without oxygen, this tissue becomes permanently damaged. The trial aims to investigate the effects of a drug that carries extra oxygen, called NanO2, on the amount of brain tissue damage. By carrying extra oxygen to brain tissue NanO2 may allow the tissue to survive for longer. It might be especially useful to prevent further damage happening while treatments to try and open the blocked blood vessel are given. Treatments may include 'clot-busting' drugs, or procedures to physically open a blocked artery. These treatments are very effective, but take time to successfully open the blockage. The study will involve treating people as early as possible after the stroke, and comparing brain scans before and after treatment. Patients diagnosed with a stroke that has occurred within the past 9 hours will be eligible to participate. The study will involve 8-15 hospitals across the UK. Participation in the study will last approximately 90 days. The majority of the study assessments will be during the first 5 days during inpatient hospital stay. There will be a telephone follow-up at 30 days and 90 days post-discharge.