An ovoo (Mongolian: овоо, heap) is a type of shamanistic cairn found in Mongolia, usually made from rocks or from wood. Ovoos are often found at the top of mountains and in high places, like mountain passes. They serve mainly as religious sites, used in worship of the mountains and the sky as well as in Buddhist ceremonies, but often are also landmarks.
When travelling, it is custom to stop and circle an ovoo three times in clockwise direction, in order to have a safer journey. Usually, rocks are picked up from the ground and added to the pile. Also, one may leave offerings in the form of sweets, money, milk, or vodka. If one is in a hurry while travelling and does not have time to stop at an ovoo, honking of the horn while passing by the ovoo will suffice.
Ovoos are also used in mountain- and sky-worshipping ceremonies that typically take place at the end of summer. Worshippers place a tree branch or stick in the ovoo and tie a blue khadag, a ceremonial silk scarf symbolic of the open sky and the sky spirit Tengger, or Tengri, to the branch. They then light a fire and make food offerings, followed by a ceremonial dance and prayers (worshippers sitting at the northwest side of the ovoo), and a feast with the food left over from the offering.