If they are isomerized you can use them at any phase of brewing, though post-fermentation would likely provide the greatest impact/lowest loss. Otoh if they are not isomerized I think you're pretty much stuck using them in the kettle to get them to disperse through the wort. Might be wrong about that but the one time I used "hop oil" I noticed much of it stuck to the high-water line of my kettle so it didn't look like using it cold-side would work at all...
Cheers!