This page covers tactics that are specific to Orcs and that proved to be helpful. Playing Orcs is fun, challenging and implies very different tactics than with other races. These tactics are applicable to WesnothMultiplayer as well as single player campaigns like SonOfTheBlackEye.
Grunts and trolls are cheap
It is easy to be tempted to recruit quick reinforcements in order to throw them quickly into the heat of battle. For orcs, this means recruiting Wolf Riders. Although they are useful to explore the map, Wolf Riders are quite expensive and have average fighting skills. In fight, they can be used sparingly to overrun enemy units and block their retreat.
On the opposite, trolls and grunts should form the bulk of orcish armies. They cost only twelve gold, that is you buy seven trolls with the same amount of gold you need to buy five riders. One orcish strategy is the "troll horde", which is achieved by recruiting tons of trolls and send them in packs, which make them difficult to stop although slow.
Trolls are very powerful units, as they regenerate. This means they recover eight hitpoints a turn. That is, a troll whelp with two HP left will totally recover in five turns! Of course, a troll won't wait that many turns without fighting, but regeneration is a good way to attack far away from villages. If your army is made up of a lot of trolls and you organize them in two groups, you'll be able to attack with one group while the other is recovering in second line. This is very interesting since non troll units have to retreat to the nearest villages to recover. Although slower, trolls are more autonomous and usually have initiative since they do not need to be organized in two lines.
Also, note that since troll regenerates, you really want to let your villages free for your other units to recover.
Trolls level up easily
Remember it, an average troll need 25XP to level up, that is two kills plus some fights. A very efficient tactic is to put a nearly advancing troll on your bridgehead. Trust me, enemy will hesitate to attack a regenerating unit, near to advancement.
Don't forget assassins
Orcish assassins are the key for every orcish offensive movement. Use them at the very begining of every offensive action. Once poisoned, a unit loses eight hitpoints a turn until cured or resting in a village. Thus, a poisoned unit can't hold a defensive position unless it is a village. If you managed to poison units, poisoning is also very efficient against units difficult to hit (like elves in forest) as poisoned units will keep losing hitpoints whether they are hit again or not.
Do not be afraid to expose your assassins as poisoned units will usually either retreat or die quickly. Of course, you need to cover your assassins with other units as although they are not that bad in close range, assassins are weak.
You might want to level an assassin or two as a second level assassin (slayer) is much more effective in poisoning and has a good close range weapon.
Orcs are kings of the hill
Orcs against undead
Against undead you should remember a few things. Most undead troops are more or less weak vs. impact attacks, so trolls are very good then.
Archers on the other hand are almost useless and assassins are even worse as undead troops (minus the Dark Adept and the Necromancer) are immune to poison, and you probably wouldn't want to attack dark adepts with ranged units anyway.
Orcs against elves
Just beware of the forest! If you fight them on grass, you have a good chance of winning. If you are forced to go into the forest, use assassins, and they have just as good defense as elves in forest, and they will poison the elves if they try to shoot you.
Orcs against humans
Mounted units receive less damage from crushing attacks (like trolls' fists) so they could kill your whelps easily. On the other hand, they are vulnerable to archers or even to assassins. Heavy Infantry are weak against assassins, because their high resistances won't prevent the 8 damage a turn dealt to them if they get hit, and they are often too slow to get to a village in time to return and be useful.
Orcs against Knalgan alliance
Dwarves are powerful fighters. They are also very resistant to blade attacks. Thus, avoid using grunts if you have to fight against them in hills or mountains. Fighters can be dealt with by a bunch of archers as they are not that immune to pierce attacks. Against gryphons, use trolls, gryphons are very weak against crushing damage. Again, the idea is (unless against gryphons): try to avoid fighting against dwarves on mountain terrain. Try to fight against other units (poachers, bandits, ...) in grasslands if possible.