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.
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.
An obstacle is something that gets in your way when you are trying to do something or go somewhere.
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.
The ocean is the vast expanse of salty water that covers most of the Earth. Another word for ocean is sea.
Scientific name: leopardus pardalis
The ocelot is a wild cat found in South & Central America and Mexico. They look similar to an ordinary cat but have markings on their coat like a jaguar. They live mainly in forested areas and hunt small animals.
An octagon is a shape with eight sides. The stop sign in the picture is an octagon, and it is mostly red.
October is the tenth month of the year. It has 31 days. The next month of the year is November. In the northern hemisphere October is part of autumn, and in the southern hemisphere it is part of spring.
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. Octopi have three hearts and blue blood. Octopi are distantly related to snails.
Something is odd if it is different to usual, or is a bit strange. In a group of things, something that is different is called the odd one out. The man in the picture is wearing odd socks. Mathematically, a number is odd if it is not a multiple of two.