Oblique: It is a type of parallel projection. It is easer to master but the crudest form of drawing in 3-D. It uses forced depth to make a two-dimentional object look like a three-dimensional object.
Orthographic: It is also a form of parallel projection but the view direction is orthogonal(perpendicular) to the projection plane. It is a way of drawing physical object with details and is mostly used in designing houses.