Orks

Tactics
Three major differences between dowpro and Dark Crusade / Soul Storm are: Fire on the Move is deadly dangerous for Orks
 * Fire on the Move
 * All Ork units are combat effective, sometimes far more so such as Slugga Boyz
 * Tactical Squads are decent in melee, given the right circumstances

It lets ranged units more easily punish mistakes by melee units: Its all about how many effective combat squads you both have
 * You can chase enemy units that are out of position
 * You can chase enemy units that are fleeing a failed fight
 * You can deal damage while kiting or falling back, with smart positioning
 * It allows you to punish pathing mistakes, such as caused by large squads of Orks awkwardly navigating bottlenecks of the map.
 * Marine armies are significantly rewarded for having more effective squads than Orks
 * They will want to negate squads, such as by breaking their morale
 * They will pick off squads that are out of position
 * They will deal enough damage to a squad so that it can be more easily handled

Orks will want to prevent the enemy from capitalising on these aims

Having more Ork squads than the enemy means either:
 * Faster reinforcement, benefiting your melee squads
 * Dealing economic damage while assaulting the enemy army, via melee squads
 * Dealing damage to the enemy while they kite, via Shoota Boyz or Trakk

Facing off against massed MEQ army from Tier 1/1.5 onwards
The three basic ork strategies are to either:
 * Create space by:
 * Raiding his base
 * Playing peekaboo at his outer points or criticals
 * Assault him suddenly in a timing window attack
 * War of attrition, where you relentlessly rely on reinforcement and a stronger economy

Know Thy Enemy
Even with just 3x Scout, 2x SM, 1x FC, there are deceptively many subtle variations to the Astartes strategy. Being constantly in his presence (due to either of the strategies above) provides intel to both sides.

Certain SM decisions set the stage for later conflict, defining: The Force Commander Can choose between: If you don't see the distinctive Iron Halo visual, you should be wary of Rapid Deployment.
 * How SM can transition
 * How you should (or should not) transition
 * Rapid Deployment (to kill your infantry faster)
 * Iron Halo (Provides ranged damage resistance, to tank your waagh banners)

3x Scouts (with Snipers), 2x Tactical Space Marine, 1x FC 3x Scouts (with Flamers), 2x Tactical Space Marine, 1x FC 3x Scouts (with Flamers), 2x Tactical Space Marine (with Flamers), 1x FC 3x Scouts, 2x Tactical Space Marine (with Powersword Sarges), 1x FC 3x Scouts, 2x Tactical Space Marine (with Heavy Bolters), 1x FC 3x Scouts, 1x Tactical Space Marine, 1x Grey Knights, 1x FC
 * Don't transition into vehicles, because SM can respond easily with Missile Launchers
 * Normally, the Big Mek can solo handle a Tactical Marine squad while a large Slugga Squad handles the other Tactical Marine squad. This is no longer advisable, as the Snipers will hurt your Big Mek, so attach your Big Mek.
 * Buy Big Mek Teleport and Big Shootas, and try to ambush Snipers who capture Critical Locations
 * Snipers do not benefit from Target Finders. They use Elite Scout Training instead. Hence this army composition is even more power intensive.
 * Buy more Orks, more for Mob Aura.
 * Contest SM on Heavy Cover.
 * Don't transition into vehicles, because SM can respond easily with Missile Launchers.
 * As it is only the Scouts taking Flamers, the SM retains the flexibility of his Tacticals.
 * Flamer scouts means SM is less likely to contest critical locations away from his main army, because they can be easily killed with Big Shootas.
 * Tougher Bosses research (Requirement: 46 pop) & Even Tougher Bosses research (Requirement: 61 pop)
 * Each gives +100 HP to squad leaders (get as many as you can).
 * Each gives +50 Morale to the Big Mek and Warboss (later transition into this)
 * Each increases the HP and DPS of Armoured Nobz (later transition into this)
 * SM will try to base bash you, and thus want Iron Halo for their FC.
 * An SM who does this may consider
 * Early Sacred Artifact Librarian with Weaken Resolve (reduces max morale and morale regen by 50% for 14 seconds, deals 5 morale damage per squad member)
 * Early Machine Cult Dreadnoughts
 * Promethium Canisters research (+15% morale damage on flamers)
 * Respond with:
 * Early Mad Doks
 * Early 2nd HQ
 * Mines
 * Transition into Vehicles, if you can
 * SM wants to use numbers to kill Slugga squads in melee. To further achieve this SM may:
 * Use Focus fire + Frag grenades
 * Get Force Commander's improved melee weapon
 * Transition into Apothecary + Chaplain, for healing auras.
 * This has the least power intensive early game for SM, therefore given a standard number of gens:
 * SM may have a Fast 2nd HQ, LP2s, Fast Tier 2, Mines or Researches (such as Frag Grenades), Fast Vehicles (such as Landspeeders)
 * Don't transition into vehicles, because SM can respond easily with Missile Launchers
 * SM will sit on Critical Locations, to get an eco lead on you.
 * Early SM Heavy Bolters may bait you to massing Heavy Infantry Orks excessively, such as Nobz or Ard Boyz, where SM can be racing towards massed scout plasma.
 * SM will probably try to smash your relic outpost. Upgrade your Waagh Banner there.
 * At Tier 2.5 Skull Probes gain Sabotage, which can disable a Waagh Banner.
 * Consider defending an upgraded Waagh Banner with a vehicle, to make SM choose wisely.
 * Transition later into Vehicles, as SM will have less anti-tank.

There are three sources of ork population

 * Waagh Banners
 * Pop Gain: +10
 * Cost: 180/15*n, where n is the number of existing waagh banners
 * Tier 3
 * Pop Gain: 10
 * Cost: 250/200
 * Requires: Boyz Hut, Pile of Gunz, 70 population
 * 2nd HQ
 * Pop Gain: 10 population
 * Cost: 250/250
 * Also provides income, more wargear, more production

Ork population grows linearly, enemy population is staggered
For Marine Equivalent Armies  For Orks
 * After Tier 1.5, population feels like it is cut in half if they heavily invest in researches and special weapons.
 * Alternatively, they either buy a population research or 2nd HQ, so army growth feels staggered.
 * Your army will feel like it is constantly growing.
 * This creates timing windows where you will have more squads than the enemy.

Major Decisions
In Tier 1, Orks have 25 population. Just past Tier 1 (around the Enemy's Tier 1.5) Orks make a choice between:
 * Waagh Banner (for 35 pop)
 * Tank Bustas (65/0/2) do not require Pile of Gunz, but do require 46 pop
 * Pile of Gunz
 * Researches & Addons
 * Stormboyz (no pop requirement)
 * Good if you're struggling to control the Critical Locations.
 * Good to raid the enemy base, as a distraction.
 * Increased squad sizes (+1 shoota boyz, +3 slugga boyz, +2 ard boyz, +1 grot)
 * If you have a few squads, this is effectively like a Waagh Banner for your total army size
 * Nobz (Requirement: Pile of Gunz and 51 pop. 80/25/3)

Tank Bustas have no power cost, therefore you can support them with very few gens. Every power gen you buy means less Orks on the field.

Nobz have a sizeable power cost, therefore you need multiple gens to support them.