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Oblate spheroid : An ellipsoid produced by rotating an ellipse through 360^o about its minor axis.

Oblique angle : An angle that is not 90^o

Oblique coordinates : A coordinate system in which the axes are not perpendicular.

Oblique triangle : A triangle that is not a right triangle.

Oblique Solid :  A slant solid geometrical figure such as a cone, pyramid or prism having axis that is not perpendicular to the base.

Oblique spherical Triangle : A spherical triangle having no right angle.

Obtuse angle : An angle larger than 90^o but smaller than 180^o

Obtuse triangle : A triangle that contains an obtuse angle.

Octagon: A polygon with 8 sides.

Octahedron : A polyhedron with 8 faces.

Octal System : A number system having the base 8 number system, and uses the digits from 0 to 7. Other commonly used number system are decimal and binary system.

Octant : Anyone of the 8 portions of space determined by the 3 coordinate planes.

Odd function: A function f(x) is called an odd function if f(x)= -f(-x) for all x in other words f(-x) = -f(x). in such functions , the left half of the plane looks like the mirror image of the right half of the plane, only upside down .

Odd number : An integer that is not divisible by 2.

Oersted :  The unit of magnetizing field strength in the CGS electromagnetic system. One oersted = 1000/ 4 π ampere-turns/meter.

One to one : A function f is said to be one to one if f(x)=f(y) implies that x=y.

Onto : A function f is said to map A onto B if for every b in B, there is some a in A such f(a)=b.

Open Curve : A curve whose ends do not meet. E.g. Parabola and Hyperbola curve.

Open set : A set U is called open if we can change any point x in U by a small amount in any direction and still be inside U.

Operator : A mathematical function such as addition, subtraction etc.

Opposite angles , sides of polygon  : Two angles , sides , vertices having equal numbers of sides between them around a polygon.

Open interval : An interval that does not include its two endpoints.

Ordered pair “ A pair of numbers in which one number is distinguished as the first number and the other as the second number of the pair

Order of Derivative : It indicates the number of times a variable is differentiated. E.g. d³x / dx³ is derivative of third order.

Order of Determinant : Determinant of an n x n square array is an nth order determinant.

Order of Differential equation: The number of the highest derivative in a differential equation.

Order of Group : The number of elements in a finite group is called the order of the group.

Order of Matrix: The order of matrix is the number of rows and columns.

Order of Root : An equation may have repeated root and order of the root is the number of times the root repeats.

Order relation : A relation, that is a criterion of comparison between objects, which satisfies the properties of reflexivity, anti-symmetry and transitivity.

Ordered Pair : A collection of two not necessarily distinct objects, one of which is distinguished as the first coordinate and the other as the second coordinate. ( a, b ) is the common notation.

Ordered Set : A set in which the binary relation is antisymmetric, transitive and also total.

Ordered Triple : The pair of numbers giving the location of a point. In three dimensional coordinate system, three numbers written in the form (x,y,z) shows the location of a point.

Ordinal number : A number indicating the order of a thing in a series

Ordinate ; The y-coordinate of a point in the plane.

Origin : The point in a coordinate plane with coordinates (0,0).

Orthic triangle : The triangle whose vertices are the feet of the altitudes of a given triangle.

Orthocenter : The point of intersection of the altitudes of a triangle.

Orthogonal : 1.Two geometrical objects are orthogonal if they meet at right angle.

2.  It is also related to a matrix whose transpose equals its inverse,

3. Also related to a linear transformation that preserves the length of vectors.

Orthogonal  Circle :  Two circles intersecting at right angle are called orthogonal circle.

Orthogonal  projections: A two dimensional graphic representation of an object in which the projecting lines are at right angles to the phase of the projection.

Orthogonal  Spheres : Two Spheres intersecting at right angles.

Orthogonal  trajectory: A family of curve in the plane that intersects a given family of curves at right angles.

Orthogonal Vectors : Two perpendicular vectors are orthogonal vectors and their dot product is zero.

Osculation :  A point on a curve where two branches have a common tangent.

Ounce : 1. A unit of mass equal to 28.35 grams. 2. A unit of capacity called fluid ounce = 1/20 of a pint.

Oval : A curve having the shape of a section of an egg.

Overdraft : Indicates the negative balance in a current account in a bank.