Equality of outcomes, typically, means very near equal income, access to resources, etc. in adult life. Regardless of their aptitude, skill, or individual choices, or even the work they do. It's a pipe dream because there's no feasible way to achieve it.
Equality of opportunity means we aim to provide everyone access to the same chances. Reduce the (present and historic) systemic discrimination by race and gender. Improve access to the basic tools required to achieve success (education and mobility). But we cannot guarantee that everyone will turn out the same (either by choice, fortune, or misfortune it just won't happen). It may still be a pipe dream, but it's far more achievable.
Why wouldn't you measure the outcomes? You have to. What you don't ask for, though, is that all outcomes be equal, because it's a fundamentally absurd proposition.
The only way that equality of opportunity would lead to equality of outcomes is if skill and preference were equally distributed.
The thing is, we know that they're not. And since skill and preference are not equally distributed, equality of opportunity will inevitably lead to inequality of outcomes. And that's okay: different people are different.
Equality of opportunity means we aim to provide everyone access to the same chances. Reduce the (present and historic) systemic discrimination by race and gender. Improve access to the basic tools required to achieve success (education and mobility). But we cannot guarantee that everyone will turn out the same (either by choice, fortune, or misfortune it just won't happen). It may still be a pipe dream, but it's far more achievable.