*Osaki ni shitsurei shimasu* can sound like an apology for leaving work before everyone else. That is not quite how it functions. It is mainly a workplace greeting that tells the people around you, “I’m leaving now.”
You would normally say it when you are leaving while your manager or coworkers are still working. This applies whether you leave exactly on time or after staying late. If other people will still be in the office after you leave, saying it before you go feels natural.
There is no need to walk around and say it to each person individually. If several people from your team are still there, saying it once so the people nearby can hear you is usually enough. The point is not to perform a separate farewell for everyone. It is simply to let the people nearby know that you are leaving.
If you are the last person in the office, there is no one for you to leave “before.” In that situation, you can just go home without saying anything.
Forgetting the phrase once or twice is unlikely to make anyone think you are rude. Leaving silently every day while other people are still working may create a different impression. It can seem slightly unfriendly or inconsiderate—not because going home is wrong, but because you keep disappearing without acknowledging the people around you.
Japanese workplaces often use greetings to mark small transitions during the day, including the moment someone finishes work and leaves. *Osaki ni shitsurei shimasu* is one of those signals.
Taken literally, it can make Japanese workers sound as though they leave the office feeling guilty every evening. In reality, they are not asking permission to go home or offering a serious apology. The meaning is closer to, “I’m heading out now. Thanks for your work, everyone.”