The one which is a covalent green oil is:

\(Mn_2O_7\)

The correct answer is option 2. \(Mn_2O_7\).

\(Mn_2O_7\), also known as manganese(VII) oxide or permanganic acid, is a covalent compound. It exists as a green, oily liquid and is a powerful oxidizing agent. The manganese atoms in \(Mn_2O_7\) are in the +7 oxidation state.

The other compounds in the options are:

1. \(Cr_3O\) - This compound does not exist in the known chemical literature.

3. \(V_2O_5\) - Vanadium(V) oxide, also known as vanadium pentoxide, is a covalent compound but is not a green oil. It is a solid with a complex structure.

4. \(CrO\) - Chromium(II) oxide is a solid compound and does not match the description of a covalent green oil.

Therefore, option 2 (\(Mn_2O_7\)) is the correct answer.