Gaining

MHC pretty much nailed it in that description, so I'll just add what little I can.

Basically the fat you eat is not the same as the fat that makes up your body. A lot of the fat you eat will end up going to things like your brain and your organs, but it's not like you're directly injecting adipose into yourself. Like MHC said it's just a longer burning fuel (which is why the ultra runners like it).

Anything you eat in excess to what your body needs could be stored as body fat whether own to you're in ketosis. The one I tell clients to watch out for (people who are trying to lose weight for some weird reason) is protein. Chemically it's very similar to body fat and any excess will be stored pretty quickly.

I hope this helps answer your question!

Nothing much to add, except that fat is more calorically dense than carbs and protein. A gram of fat has 9 calories. Carbs and protein each have 4 calories per gram.

That said, I wouldn't go chugging MCT oil in the hope that it would act as some kind of weight gain elixir. MCT oil is basically a type of refined coconut oil that is liquid at room temperature. Purportedly it provides a long-lasting energy boost and mental clarity...if you're already in ketosis. (By the way, we're talking maybe a teaspoon here at one go.) Having used it for this purpose, I would say there might be something to it, but it's hard to say how much is just hype or placebo effect.

But in general, dietary oils are poorly absorbed, and consumed in any significant quantity most likely will result in indigestion or other gastric unpleasantness.