Answer:
4
Step-by-step explanation:
When naming covalent compounds, we use prefixes to represent the number of atoms present in one molecule. The prefixes (1-10) are as follow:
Mono-, Di-, Tri-, Tetra-, Penta-, Hexa-, Hepta-, Octo-, Nona-, Deca-
Notice that osmium does not have a prefix before it. This is because it's the first element that appears in the compound name, meaning that the prefix "mono-" will not be used. However, oxygen has the prefix "tetra-", which means there are 4 oxygen atoms in osmium tetroxide.