The main strength with Orkz its all around their numbers and their Commanders/leaders. Of course, your main force will consist purelly on Slugga boyz; to hold it up well against other kind of units (specially against Eldar or Dark Eldar) it would be reccommended to use īArd Boyz in combination with Shoota Boyz.
Keep always in mind that īArd Boyz are heavy infantry, which are strong yes, but from Tier 2.5 onwards, your rival will get those plasma weaponry which will be a pain for them.
As for Commanders, it is a good choice to research as much as you can do the Teleporter for the Big Mek (canīt remember itīs name atm), which will give you a sustainable advantage and in case you are about to loose a fight (or for a fast retreat), teleport safely to another position of the map. His weapons will also prove too good against vehicles (specially his special grenade) - use them because probably is one of the best antivehicle options the Orkz have.
Last but not least, get your Waagh! banners around, the more you have, the more tech and more bonuses for your boyz you will get. The most strong bonus will offer you pluses to morale and even close combat damage.
Even though itīs kinda outdated as well, I would also reccommend you to check out the Ork manual which the mod brings. You can look at the following directory: DoWpro/DoWPro Manuals/Manuals. You will find good info about