AO, aO, Ao, or ao may refer to:
Aortic dissection occurs when a tear in the inside of the aorta causes blood to flow between the layers of the wall of the aorta, forcing the layers apart.
AIM (previously AOL (America Online) Instant Messenger) is an instant messaging and presence computer program which uses the proprietary OSCAR instant messaging protocol and the TOC protocol to allow registered users to communicate in real time.
Aortic stenosis (AS or AoS) is the narrowing of the exit of the left ventricle of the heart (where the aorta begins), such that problems result.
The aorta (/eɪˈɔːrtə/) is the main artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries).
The Aosta Valley (Italian: Valle d'Aosta [ˈvalle daˈɔsta] (official) or Val d'Aosta (usual); French: Vallée d'Aoste [vale daɔst]/[vale dɔst] (official) or Val d'Aoste (usual); Arpitan: Val d'Outa) is a mountainous semi-autonomous region in northwestern Italy.
An aortic aneurysm is an enlargement (dilation) of the aorta to greater than 1.5 times normal size. They usually cause no symptoms except when ruptured.