Oblique Drawing: An oblique drawing is a 3D drawing with the most distinguished side facing the person that observes it. It is 45 degrees to the horizontal.
Orthographic Drawings: These drawings are views ( front, side, back, etc. ) of and object, but an orthographic view is only one side.
Adapted from: http://www.cdli.ca/depted/g7/ortho.htm , and, http://toolboxes.flexiblelearning.net.au/demosites/series9/905/2_draw/draw_t3/htm/draw3_2_2.htm
Isometric drawings are 30degrees from the horizontal line but oblique drawings are 45degrees the the horizontal. Orthographic are drawings that show side of the object. Isometric drawings show three side of the figure all in dimensional proportion but oblique drawings are drawings where the most distinguished side is facing the observer.