Is the nut slip ? Or did it break inside when you applied force ?
Let the bike cool down and by using 17 number Ring pana/chabi open it anticlock wise.
If the condition of the nut is fine (i.e it edges have not become flat) than you should be able to open it.
Is the oil really coming out of the nut ? My CG's magnet seal was leaked and it looked like the oil was leaking from nut.
Change the nut if it's condition is not good.
Still if you are not able to open it i would recommend mechanic they have different tools for these types of stubborn nuts