I would say Oberlin is not competitive at all! I can’t think of any professor I’ve had that hasn’t encouraged study groups and working together, and most even recommend them. Most classes/departments don’t have quotas so you don’t have to compete for good grades, if you earn it then you get it. I’ve heard (from a friend of a friend of a friend type knowledge, so take it with a grain of salt) that upper level econ courses don’t follow this and operate more on a bell curve, but it’s still pretty cooperative.
The workload can very manageable, depending how you choose your classes. Of course some are more work than others, but as long as you don’t try to take 4 science courses with labs or 4 writing intensive courses in a semester, you should be fine! For the most part professors are also great with working with you if you have 2 tests the same day or if you need to get an extension on a paper. The P/NP and emergency incomplete policies are also great for creating a manageable workload if you realize halfway through the semester or during finals that you aren’t doing as well as you hoped.