Great question — answering as the UphillTech team:
The problem is not in your drawing - it's in the pastels. There are some media in which the quality of the materials you use make a huge difference, and oil pastels is one of them. (I also didn't like the childish way my pastel drawings looked, but getting a set of Faber Castel oil pastels changed it all.) So if you are into oil pastels, I'd suggest you get a set of good quality ones. They are expensive, but they are worth it. Also, you could use cartridge paper instead of writing/printing paper. It is the general purpose paper for artwork - it is thicker than writing/printing paper and it is not expensive.
Full answer by user18894: https://crafts.stackexchange.com/a/5775 (CC BY-SA 4.0).
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