Early Game Items
Spirit of the Elder Lizard and mercury treads are a alternative early build choice. I would get these if I was doing very well early game to snowball harder and make the enemy team fear me even more, but usually you'd want to go Spirit of the Ancient Golem and Ninja Tabi
Mid Game
Blade of the Ruined King is, in my opinion a MUST on jungle Nocturne because it works very effective with his kit. Randuin's Omen is probably my recommendation for a high armour item. Until the Runic Bulwark is removed I recommend grabbing this for your team if your support or top laner does not because it helps your team greatly in team fights.
Late Game
Your last two items should be bought depending on how you're doing. If the enemy team is building armour then you should grab Last Whisper To keep your damage high. Spirit Visage is a great item if the AP Carry is fed or is melting down your health.

--For More Information on Items please check my Notes--

Nocturne is prone to counter-jungling early game which can really make him fall behind greatly, to prevent this ask either your top laner if you are on purple team to watch for an invade or ask your support to ward your red buff if you are on blue side.

Nocturne's Duskbringer and Umbra Blades make a great clearing technique to farm some lanes or your own jungle.

When Dueling someone as Nocturne staying on your Duskbringer can greatly help with some extra attack damage, to tell if you are recieving it's buff you should see the Blue Claw as a Buff.