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.
An oar, sometimes called a paddle, is a long, thin pole with a wide, flat part at one or both ends. It is used to make a small boat like a canoe move forward, and to help in steering it.
Scientific name: avena sativa
Oats are a cereal crop grown mainly for turning into rolled oats and oatmeal for making porridge. It is also used as animal feed, particularly for horses.
An obelisk is a 3 dimensional shape with four sides, tapering towards the top. Obelisks are usually tipped with a pyramid. They are a popular shape for monuments.
You are obese if you are very fat or overweight. Obese people often have health problems because of their size.
You obey when you do what you are told to do. The dog in the picture is at obedience classes. The opposite of obey is disobey.
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.
Something is obscure if it is unclear or uncertain. Something obscured cannot easily be seen or understood. The lamp post in the picture is obscured from view.