In regards to theme, it’s possible that a reader will focus on the clash between the two main characters (the only two characters) in the piece. The primary action centers on a fight between the narrator and the woman that this narrator loves. As the two people are about to have sex, they start to argue. The conflict causes the narrator significant disquiet. He winds up in the kitchen, where he proceeds to knock everything on the table onto the floor. This includes the knife.
The theme of conflict might be developed through the knife. Its reappearance makes the characters remember that relatively tempestuous night, which is probably why the woman puts it back under the refrigerator: they’d rather not be reminded of their capacity for strife and hostility.
Another reader could pinpoint a different theme. They might think the main theme of Oppenheimer’s story is identity. Neither the narrator nor the woman are given a proper name. Their absence of a concrete name possibly plays into the theme that identity is mutable. No matter how well two people know each other, there remains a chance that they’ll act in a way that departs from their presumed identity.
The theme of volatile identity is developed by the surprise row that the two characters have in bed. The narrator notes that such a thing has never happened before. Here, the woman arguably slides the knife back under the refrigerator to avoid thinking about how identities can be subject to sudden, dramatic change.