Obelus
The division symbol ÷.
Oblique angle
Any angle that is not a right angle (or a multiple of 90°), so it is either acute or obtuse.
Oblique prism
A prism whose side edges are not perpendicular to its bases, so it leans to one side.
Observational study
A study in which researchers watch and record data without assigning treatments or changing anything.
Obtuse angle
An angle that measures more than 90° and less than 180°.
Obtuse triangle
A triangle with one obtuse angle.
Octagon
A polygon with eight sides and eight angles.
Octahedron
A polyhedron with eight faces. A regular octahedron has eight equilateral triangle faces, like two square pyramids joined at their bases.
Odd function
A function with f(−x) = −f(x), so its graph is symmetric about the origin, such as y = x³.
Odd number
A whole number that leaves a remainder of 1 when divided by 2. Odd numbers end in 1, 3, 5, 7 or 9.
Odds
A ratio comparing favorable outcomes to unfavorable ones. The odds in favor of rolling a 6 are 1 : 5.
One-step equation
An equation that can be solved with a single inverse operation, such as x + 4 = 10.
One-to-one function
A function in which each output comes from exactly one input. Its graph passes the horizontal line test.
Open interval
An interval that does not include its endpoints, written with parentheses, such as (2, 5).
Open statement
A sentence with a variable, such as n + 6 = 9, that is neither true nor false until a value is chosen for the variable; also called an open sentence.
Operation
A process that combines numbers to give a result, such as addition, subtraction, multiplication or division.
Opposite numbers
Two numbers the same distance from 0 on opposite sides, such as 4 and −4. They add to 0.
Opposite rays
Two rays that share an endpoint and point in opposite directions, forming a straight line.
Opposite side
In a right triangle, the side across from a given acute angle.
Order
An arrangement of numbers or objects following a rule, such as from least to greatest (ascending) or greatest to least (descending).
Order of magnitude
The power of 10 nearest a number, used to compare sizes roughly. Each order of magnitude is 10 times larger than the last.
Order of operations
The agreed order for evaluating expressions: parentheses, exponents, multiplication and division (left to right), then addition and subtraction (left to right). Remembered as PEMDAS or BEDMAS.
Ordered pair
A pair of numbers (x, y) that gives the location of a point on a coordinate plane. The order matters.
Ordinal numbers
Numbers that tell position or order, such as first, second and third (1st, 2nd, 3rd).
Ordinate
The y-coordinate of a point on a coordinate plane.
Origin
The point (0, 0) where the x-axis and y-axis cross.
Orthocenter
The point where the three altitudes of a triangle meet.
Orthogonal
Perpendicular; meeting at a right angle.
Ounce
A US customary unit of weight equal to 1/16 of a pound, about 28.35 grams. A fluid ounce measures volume instead.
Outcome
One possible result of a probability experiment.
Outlier
A data value much higher or lower than the rest of the data.
Oval
A rounded, egg-like closed curve. An ellipse is one kind of oval.