O button (blood type)

It stands for blood group O, the most common type worldwide. Blood is sorted into groups A, B, AB and O by the markers on its red cells. Group O has neither the A nor the B marker, and O-negative also lacks the Rh marker, which is what makes O-negative blood the "universal donor" for red-cell transfusions: it can go to almost any patient in an emergency, before anyone has had time to test their type.

That medical sense is where you will meet 🅾️ most: in a health or dating profile ("O+"), in a post from someone who just gave blood, or in an appeal next to 🩸 when a hospital or a friend needs donors.

The reason four blood types have emoji at all is Japan. The emoji came from Japanese mobile phones, and Japanese profile pages of that era typically had a blood-type field next to birthday, star sign and hobbies. In Japan, and also in South Korea and Taiwan, many people believe your blood type shapes your personality; the traits vary by source, but type O people are often called easygoing and confident, type A careful, type B free-spirited. There is no reliable evidence for it, but in Japan it is as ordinary a small-talk topic as horoscopes are elsewhere, which is why 🅾️ can turn up in profiles and "which type are you?" posts in East Asia.

What does it mean when someone sends 🅾️?

Usually either their blood type or a plain letter O; the words around it decide. In a bio or a form-style message it is almost always blood type. Inside a word it is just a decorated letter: some people spell 🅾️K, 🅾️MG or write a name with 🅰️🅱️🅾️ because the red squares stand out.

One mix-up is worth knowing: German, Italian, Turkish and Ukrainian usage, and often Swedish, writes blood group O as the digit 0, and Poles call it "group zero", so a reader from those places may take 🅾️ for a zero. It is the letter O, and it means the same group.