The Difference Between Isometric, Oblique and Orthographic

Isometric: In isometric drawing, the object is rotated 30 degrees.
Oblique: It is to produce a 2D drawing of a 3D object
Orthographic: A type of projection in which an object is depicted or a surface mapped using parallel lines to project its shape onto a plane.

Sources:
Isometric: Wiki answers
Oblique: Wikipedia
Orthographic: Dashboard Dictionary