There is one cat in the picture. The number that comes after one is two.

Photo by Hisashi_0822

Scientific name: quercus robur

Oaks are a group of trees and shrubs from the Northern hemisphere. There are both deciduous and evergreen oaks. Most people are familiar with the common oak trees that grow acorns.

Oak wood is used for making furniture, and was once used for making houses. It is still used for making wine barrels.

Photo by Brenden Ashton

Oboes are a double reed musical instrument in the woodwind family. Oboes have a very clear sound, and can be heard over other instruments. They have a conical hole through their centre, unlike the flute which has a cylindrical one.

Photo by Liberalmind1012

An observatory is a building that is made as a place to watch the stars and planets. Observatories are dome-shaped and are usually built on top of high mountains so there is less air between them and space.

Photo by Naotakem

Obsidian is a dark glass formed in the larval flows of volcanos. Obsidian doesn't have a crystalline structure and can be broken into very thin pieces that can be used to make a very sharp cutting edge, spear tip or arrow head.

Photo by Gbaku

An obtuse angle is one that is larger than 90 degrees. A 90 degree angle is a "normal" corner, like you find inside most houses and the corners of a piece of paper. An angle smaller than 90 degrees is called acute.

Photo by DanBrady

An octagon is a shape with eight sides. The stop sign in the picture is an octagon, and it is mostly red.

Photo by Greg Westfall

An octopus is a creature with eight arms that lives in the sea. The bottom of their arms is covered in suction cups. Most octopi have no bones, so they can squeeze into very small holes to hide. Octopuses have three hearts and blue blood. Octopi are distantly related to snails.

Photo by Unforth

Looking for a photo of a nice desk or side lamp, switched on and switched off from the same angle.

Oil is a substance that is liquid at room temperature. Oil does not mix with water. Oil can be heated to much higher temperatures than water can, and so is useful in cooking food. Because oil does not stick to things, it is used to help stop food sticking when it is being cooked, and it helps stop parts of machinery like car engines from sticking together when they are running. Most oils will burn under the right conditions and can be used as a heat or light source, and a fuel to run things like internal combustion engines.

Oil can be made from the seeds of many plants, as well as dug up from under the ground. Oil that comes from the ground used to be plants or animals many thousands of years ago, and had been changed into oil by the movement of the earth.

Photo by Taras Kalapun