Where are airliners oil tank?

-It will be on the engine. Each engine usually has two oil tanks, one for the engine itself and one for the Constant Speed Drive for the Engine driven generators.I am assuming you mean the fuel lubricants and engine oil. Each engine has an independent oil tank for redundancy- that way each can function if one is not functioning.



If you're referring to fuel tank, there a multiple ones. Some is kept in the center tank, under the fuselage. The rest is stored either in or under the wings. The important thing is to keep balance and center of gravity as you burn oil/fuel, which is why it is always distributed evenly and symmetrically.
Oil tanks are part of the engine accessory assemblies -

Each engine has a separate oil tank -

In the JT9D - oil tank capacity 7 US Gallons (= 27 liters) -
Engine accessory compartment.
over-wing