For those of you interested in DMing using my Alternate Combat System, here's how to make a quick conversion using Labyrinth Lord stats.
AC6, HD 1, Attack 1, Damage 1d6
First, Subtract the AC from 10 to get the equivalent AC in the Alternative Combat System. AC6 in LL = AC 4 in the ACS.
Now, AC4 = plate mail in the ACS. As orcs are subterranean creatures navigating caves and such, I don't want to give them plate mail. I'd prefer they wear leather armor, unless outfitted for war. So I'm going to give them studded leather (AC7). This penalizes them 3 armor classes from "standard", so I'm going to say that they have a natural tough skin and sheer brute-ness that increases their AC by 1(making them AC6). I'm also going to give them a -10 to their defense, as they are much more nimble than someone wearing an armored coat or similar. The -10 also makes up for two armor classes.
In the ACS, zero level characters get half their stamina for hit points, so I'm going to give all monsters an extra 5 hit points for zero level. So HD are 1+5. I'm also going to assume 5 hit points per hit die for simplicity.
As for weapon, I'm going with a long sword for the standard orc. The long sword is 0 to hit, 0 to damage.
So, standars orc stats are:
AC6, HD 1+5, HP10, CR 10, Damage 0, CP 25
So, one orc attacking another has a 35% chance to hit. A d10 is rolled with no modifier for damage, and it is indexed against AC6. It will do an average of 7 points damage per hit against AC6, or 2.45 damage per round against an equally matched orc. A battle between two such orcs should last an average of 4 rounds.
Now lets pit our first level thief, Stealthshadow against an orc.
Stealthshadow is fighting with his rapier. His CR of 27 vs the orcs CP of 25 gives him 52% chance to hit per round. His rapier will do an average of 5.5 damage against AC 6 per hit. It will take him an average of 4 rounds to kill an orc.
The orc has a CR of 10. Stealthshadow has a CR of 4. He also has a dexterity bonus of +10 which he can use each round. He can put this either in his attack roll, or he can subtract it from his CR. His strategy is going to be to avoid being hit, so he is going to subtract it from his CR, giving him a new CR of -6. The orc has a 4% chance to hit. The orc will do an average of 10.5 points damage per hit against AC8.
The orc will hit on an average of one round out of 25, and it will take an average of two hits to reduce Stealthshadow to zero hit points. However, odds are that a hit will stun Stealthshadow, and the orc has an extra 30% chance the following round to hit (+20 for stun and another 10 because Stealthshadow can't use his dex bonus), now needing a 34 or less to hit.
The odds are in Stealthshadow's favor, but a lucky swing or two from the orc can take him out quickly. Fighting orcs should not be a habit with Stealthshadow.