did that some time ago, it's not particularly nice tastewise, but you can just chug one or two shots a day - that's about 700kcal if you do 40ml per "serving" the YouTuber "DougDoug" did something similar for his diet. if you buy the cheapest oil (here it's sunflower I think) you'll get extremely many calories per unit of Money. in Germany that's about 3000 kcal per 1€

You absolutely can, if that's what works for you. Nutritionally, there's no difference between eating oil with a meal or separately. Lots of people take olive oil shots or mix butter in coffee or whatever and that's OK. That approach can be particularly good if it's helpful to think about it more like a supplement than a meal.

People may recommend against that in favor of working it into a meal you won't hate eating, and there's some merit to that for some people. But if eating is going to suck either way, just start by doing the thing that gets you calories. Work toward getting a mix of protein, fat, and carbohydrates (with potatoes + oil + eggs you have all 3) and fiber too. (potatoes have a bit) The rest can come later.

This page has a table of cheapest cost-per-calorie foods alongside some other general recommendations that may or may not be helpful. Notable items from that table are flour, bread, sugar, pasta, rice, olive oil, ramen, peanut butter, pinto beans, and lentils.

Ultimately though any food you'll eat is always better than any food you won't. A calorie is a calorie pretty much no matter what form it takes. You got this!