Oblique drawings are designed to show a three dimensional view of an object. It is a kind of a drawing that shows one face of the object in true shape, but the other faces on a distorted angle. Oblique is not really a '3D' system but a 2 dimensional view of an object with 'forced depth'.
orthographic drawing is a drawing that communicates the shape and size of an object through a series of related two-dimensional views.
An isometric drawing is a method of non-perspective pictorial drawing in which the object being drawn is turned so that three mutually perpendicular edges are equally foreshortened.
From this,I can say that the differences between these drawings are that oblique drawings are of a three-dimensional object while the orthographic drawings are of a two dimensional object and an isometric drawing is drawn in a way that three perpendicular edges are equally the same in terms of angles.