"Every company has two organizational structures: The formal one is written on the charts; the other is the everyday relationship of the men and women in the organization." Harold S. Geneen
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.
How are Organizational Units and OKRs connected?
Usually, the first instinct when creating Objectives 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 entirely optional, as you can always create Goals (Objectives) on Teams. Doing that is preferable, as Agile Tools is encouraging cross-functional teams.
Agile Tools goes a long way to become a platform, meaning that 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 your immediate use, but you can create your own.
The tool will adapt to you and not the other way around.
What is your definition of a Team?
The term Team is often used interchangeably with people/employees in a specific Organizational Unit. That is not the case in Agile Tools, where a Team is a first-class citizen, a concept that, preferably, joins people from different Organizational Units.