If you're talking about getting the last remaining bit of oud out of a bottle/vial, I like to use a 3% ambergris tincture...
I put, maybe, .3ml of tincture in the bottle, seal it, and shake it around to try and 'gather' as much oud (or other oil) from the sides and top of the bottle.
The tincture helps dilute the oud, decreasing its viscosity, making it easier to pour out... but since ambergris is an exalter, I like to think that at least some of the diluting is negated by the exalting work of the ambergris.
...or maybe you're, instead, referring to cleaning a bottle well-enough to re-use it for a different oil?