The Code of Federal Regulations (CFR), states that your time in PAID Leave status counts as “Hours of Work”. So if you have 8 hours of Annual Leave you have a sufficient number of hours to work overtime, or you can work 6 hours (regular time), use 2 hours Leave (annual/credit) and work 2 hours overtime.
REFERENCES:
5CFR 551.401 Hours of Work
5CFR 551.501 Overtime Pay
NOTE: However, you can only work Credit Hours after you have worked an 8 hour day.