Gilman's short story "The Yellow Wallpaper" deals with both oppression and mental instability. While some could justify that both are representative of the same thing, others could very easily separate them.
In order to do so, one must use the wallpaper in two very similar ways. First, the wallpaper symbolizes the oppressive nature of the narrator's husband. The wallpaper surrounds the narrator; it forces her to question her life. Second, the wallpaper symbolizes the progressive mental instability the narrator suffers from.
The narrator's pre-existing condition (what some critics have defined as postpartum depression) exists as a catalyst (something which precipitates an event)--the narrator's spiral into a more depressive state.
Therefore, one could use the wallpaper as a symbol which adds to both the oppression of the narrator and her spiriling depression.
ONe way to word this would be to state:
The wallpaper, in Gilman's short story "The Yellow Wallpaper," acts as both an oppressive symbol and one which forces her mental instability to compound.