O
Obtuse	Any angle between 90 and 180 degrees.
Octagon	Polygon with 8 sides.
Octahedron	Polyhedron with 8 faces.
Odd Number	A number that is not divisible by two.
Operations	Additon, subtraction, multiplication and division.
Order	Arrangement according to size, amount or value
Order of Operations	Order in which Operations should be done.
Outlier	a value or number that is far away from the rest.
Oval	An egg-shaped plane shape.