Examples:

obtuse angle  An angle with a measure greater than 90 degrees and less than 180 degrees.

octagon  An eight-sided polygon.

odd number  A whole number that is not divisible by 2, such as 1, 3, 5, and so on.  When an odd number is divided by 2, the remainder is 1.  A whole number is either an odd number or an even number.

ONE  See whole.

open sentence  A number sentence in which one or more variables hold the place of missing numbers.  For example, x + 3 = 5.

opposite of a number  A number that is the same distance from zero on the number line as the given number, but on the opposite side of zero.  The opposite of any number a is written as (op)a or –a.  If a is a negative number, (op)a or –a will be a positive number.  For example, if a = -5, the –a, or (op)a, is (op)-5 = 5.  See also additive inverses.

order of operations  Rules that tell the order in which operations should be performed in an expression.

order of rotation symmetry  See rotation symmetry.

ordered number pair  Two numbers in a specific order used to locate a point on a coordinate grid.  They are usually written inside parentheses, for example, (2,3).  See also coordinate.

orders of magnitude  Powers of ten.

ordinal number  A number used to express position or order in a series, such as first, third, tenth.  People generally use ordinal numbers to name dates – for example, “May fifth” rather the “May five.”

organizing data  Arranging and presenting data in a way that makes the data easier to understand.

origin  The point where the axes intersect on a coordinate grid.

outcome  The result of an event.  Heads and tails are the two outcomes of the event of tossing a coin.