Organizational Unit in Agile Tools is a hypernym for all kinds of corporate groups that accomplish a specific function. The workforce, employees, are most likely part of one such unit. The most common form of organizational chart takes the form of a hierarchical tree.
Usually, the first instinct when creating OKRs is to make them for the company, obviously, and then for each department in the organizational structure. Reconsider that if you do not want to emphasize the silo effects.
Using Organizational Units is optional, as you can always create Goals on Teams. Doing that is preferable, as Agile Tools encourages cross-functional teams.
How are Organizational Units connected to Value Units?
Agile Tools go a long way to becoming a platform, meaning you can configure many application behaviors to fit your organizational needs and preferences. For example, you can create so-called functional areas that are the foundation for creating different types of Organizational Units. Most common ones are available for immediate use, but you can create your own.