Orks have assassins in a sense. They're called Kommandos, and they come primarily out of the Blood Axes klans, though all klans have them to some extent. Less "get in, kill, get out" guys and more "Viet Cong serial killers", Kommandos move in small packs (15 boyz iz small fer orkz) and make maximum use of cover and secret passages to move ahead of the main Waaagh! and drop a load of well-equipped Orks onto a critical strategic point, stabbing, shooting and blowing up everything around them. How is this different from your average Boy, I hear you ask? Well, where a Boy will see you, charge at you with a mighty warcry and kill you with a stick, Kommandoes will sneak about, get behind your back, then charge at you with a mighty warcry, and kill you with a stick.
While most Orks hate the idea of sneaking around and for the most part despise Kommandos, Kommandos are still considered invaluable for their reconnaissance work and ability to be exactly where the enemy most wants them not to be. Their natural inability to be subtle and monstrous size relegates the Kommandos to in-field operations, and rarely do they cross into civilian territory. This also means that Kommandoes get to make sure the main WAAAGH blasts the right things at the right time, as the only "eyes and ears" of the Orks are themselves.
Want to know how cunnin' they can really be? One time, WAAAGH! Grog, a small ork WAAAGH!, was having trouble out-dakkaing a Tau battleship guarding a Tau planet, the Korst'la (the Orks literally called it "The Big Dakkaship"). While Grog was off trying to get some more dakka, and hopefully this time maybe smash the thing, some Ork Kommandos figured out it would be easier just to smash the only spaceport the Korst'la could dock at. They took a captured Tau transport ship, and literally bullshitted their way into the docks ("Help us, mighty Ethereals, help. Ork pirates everywhere zogg-I mean, pummeled our fleet really bad. Please, we need repairs fast.") AND IT WORKED. You can imagine the epic slaughterfest that ensued once the ship's hatches opened. Station went boom, The Big Dakkaship soon became The Big EmptyFueltankShip, and WAAAGH! Grog proceeded to assrape the entire sector. If this sounds familiar then congratulations, you know your history.
However, certain Kommandos of note, most prominently Snikrot and his Red Skull Kommandos, are most certainly assassins in the traditional sense, moving silently and infiltrating just below their victims noses before slitting their throats, taking a trophy and, in Snikrot's band's case, scalping them. Despite this, he is not the most famous Kommando. That would be Spookums, he who can total everything with a Rokkit Launcha and a grappling hook. He also tried to camouflage himself in lava at one point in his career.
Ork Kommandos can be bought in mobs of 15 like most special Boyz, and cost 10 points each. So, 4 points more than your average Slugga Boy, in smaller mobs... What could they possibly bring on the table?
Well, Orks who can make it to the front line without losing the entirety of the mob.
Kommandos come with Stikkbombs like everyone else in the new 7th Codex, and has the Special Abilities Infiltrate, Move Through Cover, and something new: Stealth, which actually make them quite capable of coming in from a flank or dodging through the rubble before charging their enemies with glee. Mind you, they are the same Boyz as normal Sluggas with respect to their statline, and they get the exact same upgrades as well, with the exception of the option to get a Burna in the Mob and losing out on the option for 'Eavy Armour - treat them as such, while remembering that they can and will utilize Cover a lot better than all other dudes in your army, save for Grots.
They are generally a good choice for any Ork army, and can do very well with a bit of low kunnin', but don't expect them to perform miracles just because of their Special Rules. they can be killed by all the same things, and if your opponents bring Flamers, you might as well take a regular mob of Boyz instead, - they'll fare just as well.
A bonus they also get is the ability to take Boss Snikrot (mentioned above) as a free character, allowing him to join their force. Doing this allows for Snikrot to join the Kommando mob from reserves and pick any side to infiltrate from. To make it even better, they get Shrouded instead of Stealth the turn they arrive, giving them a way to survive fire until they can get to the killing.
In 8th Edition, Ork Kommandos can now Deep Strike 9" away from enemies anywhere on the table, as can Snikrot (They aren't technically deep-striking, just coming out of hiding, but for intents and purposes its the same thing). They also get a 4+ armor save from ranged when in cover. Snikrot gets a 3+, effectively making him a space marine when in cover. Snikrot also now has a re-roll 1's aura of six inches, allowing him and a "Strike Force" of 15 Kommandos to pop up with pretty decent chances of making a first-turn charge. This is great against Tau or Guard armies, where squishy important units often are hidden way in the back, where its hard to reach them with conventional foot-sloggers.