Serums are generally viscous fluids that are concentrated in active ingredients. Thus, they're like concentrates I think. What serums do will depend on their formula and intended purpose. There're serums for acne, serums for wrinkles, serums for pigmentation, etc. So you'll have to find something that suit your skin. Hmm...a serum for acne that is basically just oil? That sounds scary indeed. Did you check the ingredient listing to find out what kind of oil does it use? Anyway, if you've acne prone skin, it's better to stay away from oils, including plant oils(with the exception of jojoba which I believe you can use sparingly for facial massage).
A serum is a liquid with active ingredients like vitamins and anti-oxidants that are chemically formulated so they can penetrate deep into the skin. Serums are designed to penetrate and treat various skin conditions, redness, dehydration or lack of firmness. They can be applied in the morning or night or both.
There are quite a few oils that won't aggravate acne, or cause a flare up. Here are just a few. Tea tree oil, Kukui nut oil, Borage Seed oil, Lavender oil, and emu oil. I am sure there are many more. Just because you have acne, doesn't mean you can't use a facial oil or that you have to stay away from facial serums. They actually might be a good thing for you- coming from an acne suffer myself-
then you're right, you shouldn't be using it on acne-prone skin. It says "anti-ageing" and contains safflower oil. You would do better with an oil-free serum, if you really want a serum. Stay away from silicone, dimethicone and all the 'cones which can make you look greasy.
A serum that would be really nice for you would be MyChelle: