Opportunity cost is the cost of the opportunity that disappears if you go one way and not the other, and we all must make choices in our life. For example, someone gets into an ivy league college, which has been their dream for many years, but that same person also just accepted a high paying job.
This person must look at the two choices and weigh the pros and cons of each option. The answer is not identical for everybody, though. It all depends on what you value, and what your priorities are in life. On the other hand, tradeoffs are the options we give up obtaining a specific outcome, experience, or service that we want.
It is an agreement that comes in the form of a compromise. To say, give this up for what you wish. Of course, it means that we have to lose something when making the decision—choosing which one to tradeoff is often not an easy thing to do.