Right. Seeing as we only really have one build order posted up by anyone (ND's Eldar) I thought I'd add to the list. Vs Marines and Chaos Marines in Automatch I've had reasonable success thus far. Unfortunately, if you're even begin a game vs Eldar you might as well forget it. Orks can't win when the sides are of even skill.
Now then, onto the build order.
Step 1: 1st Grot unit builds Waaagh banner. Queue 2nd grot unit and a Slugga unit (asap) at stronghold. Once the 2nd grot is built, get him to help on the Waaagh banner. Put both grot units on Overwatch. 1st slugga unit caps the strategic point closest to you but furthest from the enemy. DO NOT GO FOR RELICS OR CRIT LOCATIONS EARLY.
Step 2: Both grot units build Da Boyz Hut, both slugga units proceed to capture in the same fashion as before. Queue 3rd Slugga unit at stronghold.
Step 3: When Boyz Hut is finished, build a Big Mek there and 4th slugga squad at stronghold. Grots build LP on first captured point.
Step 4: Group the 4 squads and the big mek, build 3rd Waaagh banner and a power plant followed by a Pile O Guns with the Gretchin.
When attacking with your mob of orks, remember to stick them all on Overwatch and use the Pause Overwatch button to control the rate of reinforcement, otherwise you'll use up all your req too fast and won't be able to tech. This force is able to overrun most early Marine/Chaos Marine forces. Go for base if possible. Target priority is:
2: Servs/Heretics building Turrets
3: Marine Squads
4: Scout Squads
5: LPs followed by Decap.
7: Barracks and Power
If you encounter a fairly resisitant force on the way, hit the character first, followed by large marine squads. Ignore scouts for now, most players will sit them with either flamers or snipers. By this point, try to have burnas and Nob Leaders in your squads, and have researched Kustom Tellyporta and Kustom Force Field. Big Shoota or Rokkit Launcha upgrade research is also helpful when trying to tech fast to Vehicles - Wartraks are your best friend. Keep building LPs on your captured points. Good luck!