oblique angle: An angle that is neither a right, acute or obtuse angle.
obtuse angle: An angle whose measure is greater than 
obtuse triangle: A triangle with an obtuse angle.
octagon: A polygon with 8 sides.
odd number: A whole number that is not evenly divisible by 2 .
odds: The ratio of the probability that an event will occur compared with the probability of it not occurring.
operation: Addition, subtraction, multiplication, and division are the basic arithmetic operations.
operator: The symbol that expresses the operation to be performed.
opposites: Two numbers that are located the same distance from 0 on the number line but in opposite directions. The sum of opposite numbers is 0 .
ordered pair: Set of two numbers in which the order has an agreed-upon meaning, such as the Cartesian coordinates (x, y) , where the first coordinate represents the horizontal position, and the second coordinate represents the vertical position.
order of operations: A set of rules for the order in which to solve mathematical problems. Start by multiplying and dividing, then adding and subtracting, always working within parentheses first.
ordinal number: A number used to indicate place or position within a set or group.
ordinate: The second element in a coordinate pair. When graphed in the coordinate plane, it is the distance from the x-axis. Frequently called the y coordinate.
origin: The point (0, 0) on a coordinate plane, where the x-axis and the y-axis intersect.
outcome: In probability, a possible result of an experiment.
oxidation state: An indicator of the degree of oxidation of an atom in a chemical compound.
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