In the purest (type designer) sense, an oblique is a roman font that has been skewed a certain number of degrees (8-12 degrees, usually). An italic is created by the type designer with specific characters (notably lowercase a) drawn differently to create a more calligraphic, as well as slanted version.
Some type foundries have arbitrarily created obliques that aren't necessarily approved by the designers themselves... some fonts were meant not to be italicized or obliqued... but people did anyway. And as you may know, some operating systems will, upon clicking the 'italic' icon, skew the font and create an oblique on the fly. Not a pleasant sight.
It's best to specify an italic only when you're sure that font has been designed with one.
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