If I gave someone Org Access of "All", why can't they see all people?
There are 3 possible reasons for this:
1) In reports, if the person has the appropriate roles and Org access, then it could be that when they run the report they choose "All People I can see" instead of "My Primary Approvees".
Navigate: Reports > Saved > Edit Pencil Icon.
2) Separately - In reports or grids, it may be that the project's Access tab has been unchecked for Org Access. (If unchecked, no access granted via Org Access will be allowed and access may only be granted via Project Administrators.) Best practice is to always leave these boxes checked so they do not interfere with granting Org Access in the person profile. You may want to uncheck them if you have a top secret project or otherwise want to tightly control access.
Navigate: Project > List > List > Edit Pencil Icon > Access.
3) Be sure that you granted "All" org access to the "Person" type of Org Access. Note that there are Person, Project and Owning types of Organization Access. Person type controls which people they can assign to those projects. Project type controls which projects they can assign to, Owning type controls which projects within a project owning organizations they can assign to.
Navigate: People > List > List > Organization Access.