Is a wet-mount needed to use Immersion Oil?

When I prepare my slide to use immersion oil, do I really need to wet-mount the specimen? Can I do it with just the coverslip?

Ideally you shouldn't use either with immersion oil. You should match the mounting media index as closely as possible to the immersion oil index, otherwise you'll reduce the resolution and contrast of your image. See:

http://www.microbehunter.com/the-importance-of-mounting-medium-refractive-index/

Using water for the wet-mount gives you a sort-of time limit. You have to do your work before the water evaporates else you get little rivers at x100.

There are lots of options for higher index mounting media that will also preserve your specimen at least as well as air.

The answer is that you cannot use the Immersion Oil without water-mounting the coverslip.

I tried it.

The water-mount "glues" the coverslip to the slide, effectively anchoring it in place. Without doing this, when you add the Immersion Oil, the coverslip will slide around because the oil "grabs" the coverslip. When you pan the stage, the coverslip is stuck to the lens and the slide on the stage is moved around but the coverslip doesn't move because it's adhered to the lens.

When you water-mount the coverslip, it is anchored so solidly that it is actually even difficult to push it with your fingernail.

Conclusion: No Immersion Oil without a water-mount!