Main Difference
The main difference between Oval and Ellipse is that the Oval is a shape and Ellipse is a type of curve on a plane.

Oval
An oval (from Latin ovum, “egg”) is a closed curve in a plane which “loosely” resembles the outline of an egg. The term is not very specific, but in some areas (projective geometry, technical drawing, etc.) it is given a more precise definition, which may include either one or two axes of symmetry. In common English, the term is used in a broader sense: any shape which reminds one of an egg. The threedimensional version of an oval is called an ovoid.

Ellipse
In mathematics, an ellipse is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve. As such, it is a generalization of a circle, which is a special type of an ellipse having both focal points at the same location. The shape of an ellipse (how “elongated” it is) is represented by its eccentricity, which for an ellipse can be any number from 0 (the limiting case of a circle) to arbitrarily close to but less than 1.
Ellipses are the closed type of conic section: a plane curve resulting from the intersection of a cone by a plane (see figure to the right). Ellipses have many similarities with the other two forms of conic sections: parabolas and hyperbolas, both of which are open and unbounded. The cross section of a cylinder is an ellipse, unless the section is parallel to the axis of the cylinder.
Analytically, an ellipse may also be defined as the set of points such that the ratio of the distance of each point on the curve from a given point (called a focus or focal point) to the distance from that same point on the curve to a given line (called the directrix) is a constant. This ratio is the abovementioned eccentricity of the ellipse.
An ellipse may also be defined analytically as the set of points for each of which the sum of its distances to two foci is a fixed number.
Ellipses are common in physics, astronomy and engineering. For example, the orbit of each planet in our solar system is approximately an ellipse with the barycenter of the planet–Sun pair at one of the focal points. The same is true for moons orbiting planets and all other systems having two astronomical bodies. The shapes of planets and stars are often well described by ellipsoids. Ellipses also arise as images of a circle under parallel projection and the bounded cases of perspective projection, which are simply intersections of the projective cone with the plane of projection. It is also the simplest Lissajous figure formed when the horizontal and vertical motions are sinusoids with the same frequency. A similar effect leads to elliptical polarization of light in optics.
The name, ἔλλειψις (élleipsis, “omission”), was given by Apollonius of Perga in his Conics, emphasizing the connection of the curve with “application of areas”.

Oval (noun)
A shape rather like an egg or an ellipse.

Oval (noun)
A sporting arena etc. of this shape.

Oval (noun)
In a projective plane, a set of points, no three collinear, such that there is a unique tangent line at each point. (A tangent line is defined as a line meeting the point set at only one point, also known as a 1secant.)

Oval (adjective)
Having the shape of an oval.

Oval (adjective)
Of or pertaining to an ovum.
“oval conceptions”

Ellipse (noun)
A foci of the ellipse) is constant; equivalently, the conic section that is the intersection of a cone with a plane that does not intersect the base of the cone.

Ellipse (verb)
To remove from a phrase a word which is grammatically needed, but which is clearly understood without having to be stated.
“In B’s response to A’s question: (A: Would you like to go out?, B: I’d love to), the words that are ellipsed are go out.“

Oval (adjective)
having a rounded and slightly elongated outline or shape like that of an egg
“her smooth oval face”

Oval (noun)
a body, object, or design with an oval shape or outline
“cut out two small ovals from the felt”

Oval (noun)
an oval sports field or racing track.

Oval (noun)
a ground for Australian Rules football.