Be working overtime
Definition 1 of 2
Pro Tip 1/3Beyond Standard
Refers to working past the usual closing time or agreed hours. 
He's been working overtime to meet the project deadline.
Pro Tip 2/3Possible Compensation
May imply extra pay or time off, commonly known as 'overtime pay'. 
She earns extra by working overtime during the holiday rush.
Pro Tip 3/3Extra Workload
Implies an increased workload that necessitates longer working hours. 
Since the merger, our team has been working overtime.