The classic wage, schooling, and ability trap
The true model is (centered, exogenous). Ability is unobserved, so you regress wage on years of schooling alone.
Derive what your estimated return to schooling converges to, and sign the bias.
Show a hint
Ability plausibly raises wages on its own, and more able people tend to get more schooling. Feed both facts into the OVB formula.
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.