Fuming sulfuric acid, often known as oleum, can be represented chemically using the formula $ H_2S_2O_7 $. This compound is formed by adding excess sulfur trioxide ($ SO_3 $) to sulfuric acid ($ H_2SO_4 $). The chemical reaction can be represented as follows:

$ H_2SO_4 + SO_3 \rightarrow H_2S_2O_7 $

From the formula $ H_2S_2O_7 $, to determine the number of oxygen atoms, we look directly at the subscript corresponding to oxygen in the chemical formula. In this case, the subscript is 7, which indicates that there are 7 oxygen atoms present in the molecule of fuming sulfuric acid.