OB/GYN residency is really tough. It's probably one of the toughest residencies. Why? 1) it's four years (not three like most), 2) it's mostly in-hospital, which means you frequently on call, 3) it's high stress because every time you go into delivery, 2 lives are on the line, and 4) it doesn't get easier to years 2 and 3, and it's a high stress environment.
You work really really hard in residency. You might do rounds in the morning, making sure everything is in order for your patients. When you are a OB resident, you make sure the patients going into labor and those who have just delivered are taken care of. When you are a Gyn resident, you make sure all of the patients are covered, before and after operations. You have to really know your patients, e.g., you take lots of notes, you know their lab test results, you know how they are doing after operations, you know what their diet should consist of.
Check out this video, which takes you along during the day of an OB/Gyn rotation at Harbor Hospital with a AUA medical school student: