This is a freewheel right? The cassette screws onto the hub(?)

If so, you can, like others mentioned, drip some lube down into the crack between the lockring and the freewheel body OR you can disassemble it. Sheldon Brown has a procedure to do this - here.

The "drip in the lube" method is what I had done in the past to my old winter bike. But I had taken the freewheel apart to see the guts of 'er; cleaned it up, regreased and remounted and it worked fine until I replaced the wheels with a freehub.

It is a good way to see the innards of the freewheel and an educational component as well. However, it would be less trouble to just buy a new freewheel.