From what I've read (in places such as
this page (http://www.richardspens.com/?page=ref_info/nib_primer.htm) on Richard Binder's informative web site), an oblique nib enables you to write with the pen held at a different angle (possibly a more comfortable angle) in the hand rather than changing the style of writing or the angle of the nib edge on the paper. You need to make sure you get the correct angle (right or left oblique) for your style of writing - it doesn't help that the naming seems inconsistent between some manufacturers, and some apparently don't even specify the angle.
Depending on the grip/section, you can still twist the pen to change where the thick/thin sections are - the italics books I've read mostly seem to recommend holding the nib edge (whether normal italic or oblique italic) at 45 degrees to the paper vertical/horizontal, so thin sections are on the rising diagonal rather than on horizontal strokes. Of course, there's no reason anyone's writing necessarily has to follow these recommendations if they prefer thin horizontal strokes.
Regards, Myles.