So, both Orks and Chaos Space Marines are, relative to the Imperium or Eldar (all types), a bit on the back foot competitively right now and also have pretty limited access to allies to make up for their weaknesses.
Something that not a lot of people realize is that Orks and CSM are actually Allies of Convenience, meaning that while they cannot share ICs and Psychic Blessings, they also do not have any real negative consequences for working together as allies. Somewhat surprisingly, I think these two factions actually bring a lot to the table in terms of synergizing with one another.
Here are a few ideas I have right off the top:
1 – Cheap Hordes — While CSM are relatively affordable (starting at 12 PPM), they are still paying MEQ points for their basic troops (Cultists aside). Orks can literally flood the table with T4 bodies that are dangerous in CC, in turn pulling enemy combat power away from the more pricy CSM units. Also, since almost everything except Meganobz have a 6+ armor save, Orks provide a great meat shield/distraction against Grav-heavy Space Marine armies that normally are so dangerous to CSM.
2 – Mega Force Field — This is a Relic from the Ghazghkull supplement for a Big Mek that projects a 4++ Invul save against shooting attacks to ALL models within 6″ (this means that even enemy units get the save, which applies to allied CSM, who count as “enemy” models per the Allies of Convenience rules). There are lots of applications for this with CSM allies, such as giving Chaos Land Raiders or Spartans a 4++ to help them get across the battlefield and deliver their assault troops successfully, buffing nearby CSM with Mark of Tzeentch to a 3++ save against shooting, providing Cultists camping on objectives with a 4++, etc. etc.
3 – Mek Gunz Fire Support — These are one of the hidden gems of the Ork codex, I think. With T7, a 3+ save, and multiple Gretchin crew per gun (and the ability to take up to five guns per battery), they are surprisingly survivable and can put out some very potent firepower to supplement CSM allies, despite Gretchin BS3. The Kannon provides a versatile Missile Launcher equivalent with both Frag and Blast shells, the Lobba and Bubblechukka provide anti-horde capacity with their blasts, the Zzap Gun and Smasha Gun provide potential anti-armor/anti-TEQ with their low AP shots, the Kustom Mega-Kannons are basically S8 AP2 Plasma Cannons, and the Traktor Cannon gives cheap and highly effective AA defense to CSM forces that often lack this.
4 – Tankbustas — Another little-known gem from the Ork codex, Tankbustas are quite solid against a variety of targets, despite their BS2. With 10 Boyz in a Mob they can reliable get 3-4 hits with their Rokkit Launchas per turn. Add in Tank Hunter USR for added power against vehicles and easy access to “bunkers” in the form of Trukks or Battle Wagons from which they can shoot all their weapons (thanks to their Open-Topped nature) and they provide massed S8 AP3 firepower at a very economical points cost to deal with enemy MCs, vehicles, and medium infantry, all of which often requires much more expensive options in the CSM codex.
–CHAOS SPACE MARINES–
1 – Psychic Might — While Orks have Weirdboyz, they are limited in numbers, PML, and available disciplines. With the advent of the Cyclopia Cabal, CSM now can get 3-5 Sorcerers (all of which can get up to PML(3) easily)), all of whom can access Chaos disciplines plus Biomancy/Daemonology/ Telepathy plus their unique formation ability “Shroud of Deceit,” which enables you to take control of an enemy non-vehicle unit and shoot with it at your opponents (obviously, this can be an amazing help to Orks, especially when facing firepower heavy opponents like Eldar, Tau, and Astra Militarum).
2 – Dirge Casters — Since Orks are really geared to getting into assault in large numbers, anything that helps them do that and minimizes enemy interference is a boon. Enter the CSM Dirge Caster. Shutting down Overwatch by any enemy units within 6″ of the vehicle equipped with the Dirge Caster can make quite a difference for the poorly armored Orks, especially when charging armies like Tau or Dark Angels that have significant “buffs” to their Overwatch abilities.
3 – Heldrakes — While Orks have multiple flyers of their own, none of them have the raw power, survivability, and anti-infantry damage output (at least consistently) of one or more allied Heldrakes with Baleflamers. Able to come in 36″ and then torrent another 12″ with their S6 AP3 Ignore Cover Templates, you can reliable kill entire squads of key enemy units in a single turn with these weapons. In terms of supporting allied Orks, this can be critical for taking out key enemy non-vehicle units (i.e. Long Fangs, Eldar Warp Spiders and Jetbikes, Tau Pathfinders, AM Heavy Weapons Teams, Necron Destroyers, SoB Retributors, Skitarii Vanguard, etc.) that would otherwise be devastating to Ork forces.
4 – Tough Objective Holders — Since Orks are so lightly armored and can have leadership issues when not made Fearless by special rules/characters, they can struggle to hold objectives. On the flip side, CSM have access to Plague Marines, which are not only Fearless, but have T5 and FNP built in, making them great for holding those Objective late game even (especially if they are made Troops and gain Objective Secured) when your opponent levels his entire arsenal at them. Alternately, if you have Typhus as a HQ, you can then take super-cheap Plague Zombies with Fearless/FNP for the ultimate in cheap, tough, unshakeable Objective Secured units.