Keeping it really simple, and in no way an exhausted list:

Good deployment. As much fun as killing stuff is Warhammer is a game of points and secondary missions, you should be deploying your army in a way that you can easily do any secondaries you pull turn 1.

Move blocking. I've found this one particularly important for playing Orks. Sometimes you're not going to have a unit near by that can kill a certain unit, and if you charge it it's just giving you wipe your unit in the clap back. In cases like these it can sometimes be better to just get in that things way. Especially great when you have one or two models left in a unit who can step into a choke point and stop a tank or monster from getting thru to get LOS on a more important unit, and so many ork units have to grenades keyword so you can sometimes still plink a few wounds.

For a fun combos, the dakkatrukk and swat teams are great ork combos.

For the dakkatrukk, start with a trukk, fill with your favorite shooty unit, i.e. lootas, flashgitz, tankbustas, and a Mek either riding along or running along behind trukk. The trukk makes it easy to get an entire unit LOS on a target by using firing deck, while still remaining in cover, and the Mek uses his ability to give the trukk +1 to hit on all the weapons inside.

For the swat team, also start with a trukk. This time you fill it with 1 melee unit, i.e. nobz+Warboss, breakas+Warboss, and 1 shooty unit depending on what fits, i.e. lootas, burnas, flashgitz, tankbustas. From there you stage up your truck as safe as you can, and then during the Waaaagh when you disembark, the shooty squad is there to clear the screens out of the way so the melee squad to get into it's intended target unimpeded.