"Ojama shimasu" literally means, "I'm going to get in your way" or " I will disturb you." It is used as a polite greeting when entering someone's home. ... This expression is used in various situations. When entering someone's house or room, it means "Excuse my interrupting."
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This set phrase is said whenever you enter someone else’s house, signifying that you know you are going to be a bother and apologize in advance. When using this phrase, you are signifying your own modesty and sense that you are intruding.