To most people, diagonal and oblique mean the same thing. For compositional purposes, though, diagonal can mean a line that runs between opposite corners of a shape (such as the image frame). Oblique can describe a line that intersects the middle of another line, instead of the corner of a shape.
Oblique lines draw the eye more than horizontal or diagonal lines. This is because lines that are parallel to the picture frame are more static than those that slope against it. The image below contains several oblique lines.