No—it is not okay for any employee to work for equity only. All employees should be paid at least the applicable minimum hourly rate for all hours worked, including overtime at the applicable overtime rate for any overtime worked. To the extent the company wants to classify a US-based employee as exempt from overtime, it needs to pay the employee at least the applicable minimum salary for exempt employees. Should your company not presently have sufficient capital to pay wages, please consult with your employment lawyer as to possible options for addressing this issue once the company has sufficient capital.