The Fair Labor Standards Act and its regulations govern exempt/non-exempt status and overtime requirements. The law sets a minimum salary employees must be paid to be considered "exempt" from overtime and also has several "duties" requirements that must be met for a job to be considered exempt. Jobs that manage other employees or that require a high level of "discretion and independent judgment" qualify for the exemption if they meet the salary threshold.

I'd say it's probably unlikely an office manager would satisfy the "duties" test unless he/she is directly responsible for managing other employees for the majority of their time.

Depending on the state, there may be specific state law requirements - like a higher salary. Unfortunately these laws are complex. It's good to get some advice from an attorney if you want to mitigate risk.

Here's a guide that the Department of Labor has published. Happy reading. Haha

https://www.dol.gov/sites/dolgov/files/WHD/legacy/files/Digital_Reference_Guide_FLSA.pdf