Orcs are known as the most warlike of all races though they usually lack the discipline and organization one would assume from a standing army. Instead a common pack behaviour is known to dominate their decisions on the battlefield showing only a minimum of coordination among one clan. Thus it is nearly impossible to put orcs from different clans or even tribes into one fighting unit because they'd rather turn against each other for honour and loot than fight the enemy. On more than one occasion disputes among rivaling clans and leaders have saved the day for a human, elven or dwarven army and made close to defeats glaring victories.
Nevertheless though looked upon by all other races orcs in battle are not respected as equal in any way but certainly they're feared.
To overcome the lack of a central leadership, a general of sorts, some tribes - like the Losh-oc or Ashz-oc - at least agreed to elect warlords for certain times and a certain task. However these warlords are still not seen to be equal to a general. In fact they're more the head of a war council consisting of all clanleaders which means that in the end they still can only depend on the warriors of their own clan. In even greater campaigns these warlords lead their own armies to the battlefield and again form some kind of council to decide what to do together.
While this system has proven to be surprisingly stable it is also the reason that orcs never make any clever battle plans. In the vast hierachies of clan relations and alliances it would be nearly impossible to force one clan to stand back as some kind of relief force or sneak up from behind while others gain all the fame. This has also proven to be the key weakness of most orcish campaigns because when fighting in their very own clan they are nearly unmatched in the way they fight together.
Organization: The orcish way of war is very clan-based. The warriors of one clan are generally the smallest orcish fighting unit which consists of between 50 to 300 warriors. This is usually enough to commence raids, scouting and smaller engagement but for a fullscale campaign one clan is generally too weak so several clans ally for a common goal and rally their troops.
For this purpose warlords are elected. They already lead whole armies of 1000 to up to 6000 warriors and command them in agreement with the participating clanleaders. In even greater campaigns, where different parties join for a common goal these warlords only coordinate their moves in councils but stay mainly independent from each other. Such occasions are only very rare, the last time such a huge orc army has been seen marching together in the south was in the Third Gaian War. Sigh.
Otherwise it can be said that you have various orcish armies that only contact each other about their plans when necessary and coordinate attacks only when the odds force them to do so.
Tactics: The smallest unit among orcs represent their clans where family bonds force them to stay rather disciplined and they're willing to watch each other's back in battle.
A clan usually attacks in a triangular formation to break the enemy line as quickly and as lethal as possible. Orcs - except the Osther-oc - don't have a great feel for battle formations and subtle tactics. Their favourite battle plan is an overwhelming frontal assault that either breaks the enemy lines or ends in many small skirmishes. This would be an easy to dodge situation for any well-trained army when the different clanleaders wouldn't lead their warriors independently on the battlefield, everyone of them seeking a good opportunity for their attack. For the opposing side this means that orcs usually hit them in waves from different sides, at different angels, in different strength endangering any battle formation to fall in total chaos, a situation orcs know best to exploit.
While there are no exercised battle formations most orcish clans nevertheless follow a certain order in battle.
The Skirmishers
The first line of orcs are usually Skirmishers with only spears and clubs to fight. They consist either of very poor or rather young warriors. They're meant to break the enemy line just long enough to give the following orcs room to exploit opportunities.
The Shieldbearers
The Skirmishers are followed by the Shieldbearers, orcs with wooden shields and either axes or swords. They'll draw the enemy in many small fights and will widen any gap that might have opened in the enemy's line.
The Pike and Spear Orcs
They're then followed by orcs with pikes and spears. Those orcs will push back the enemy with their long pikes and eventually cover the orcs in front of them from any quick counterattacks.
The Elite Warriors
The last line usually represent the elite warriors, orcs with sword, shield and bow, often clad in more or less effective armor, sometimes accompanied by archers with only shortbows. While the archers will just provide cover from behind the lines, the Elite Warriors will support their front men and will stand ready anywhere where the own line might break. They're the last to come to fight but they're meant to fight any serious resistance that has survived the onslaught from the other lines with their expertise and better weapons.
Classes: The Gaian orcs share very similar categories of warrior classes among the different tribes, supposedly as they all share a common language and all have a common heritage. Some tribes will lack certain classes due to limitations in build or in equipment but nevertheless they at least know the purpose of certain warriors:
Noruck-chmer (Skirmishers)
Only equipped with simple weapons like clubs, spears and dagger the "Norucks" are the first line in a common battle. They usually wear no or only light armor in the form of hides or leather and consist of either very poor orcs or young warriors that have to prove themselves.
They'll either lure the enemy into a fight or break the enemy line to make room for the older and better equipped ones. The Norucks are very common among the Ashz-Oc and Losh-Oc.
Gorucks (Shieldbearers)
The Gorucks can be considered as the orcish standard clanwarrior. Equipped with a sturdy wooden shield and a slashing weapon of considerable strength - either a sword of sorts or an axe - they are already a worthy opponent to any infantrymen on any battlefield. Sometimes the Gorucks already wear some armor pieces but their strength lies in adequate weaponry combined with great agility. Most orc warriors consist of this type, they're orcs who have proven their worth in combat. The Osther-Oc have thes Gorucks equipped with heavy armor otherwise not known to other orc tribes and all equipped with broadsword or battleaxe.
Ashzuck (Worthy Warriors)
The Osther-Oc, Ashz-Oc and Losh-Oc are considered the "Great Orcs" in Sarvonia as they reach nearly the height of man. Thus they make formidable "Gorucks" and "Ashzuck". The "Ashzucks" differ mainly in status from the normal soldiers, the "Gorucks". They're veterans and usually among the eldest of the clan. As an orc does not grow weak with age their combat skills do not suffer from it, instead most of them have enough foes and loot from victories to put together a very good equipment. Very often they have a sword, shield, spear and even bow and clad themselves in scale or chain - rarely in plate - mail. Only they are usually wealthy enough to afford an orcish warbow and they're known to use it well. The family and clanleaders are usually of this class with enough wisdom and experience to know where and how to fight.
Ngangas (Archers)
These are orcs who only fight with their hunting bows and a knife for selfdefense. They're looked upon by the warriors as they don't even fight in the first row of the battle. Thus they're not common among the "Great Orcs" - though Osther-Oc might train and deploy some for various tasks. Smaller orcs, those living in the wilderness however have been known to often participate in campaigns of the "Great Orcs" as such archers as they would be totally overpowered in the first battleline.
Rochock (Wargriders)
Only the Rhom-oc are known to deploy wargriders in vast numbers, in fact all of them fight from the warg's back. In many other tribes the Rochock are more of a myth as they do not breed wargs which are large enough to carry an orc. Wargriders are usually more scouts and mounted archers than anything else. While they can be formidable against infantry orcs depend more on their sturdy pikes and arrows when they move against human or elven cavalry.
Beside the Rhom-oc only the Losh-Oc are known to use wargriders more often as the Losh-Oc have bred the Warwarg, a warg species as big as a horse.
Morgur (Wargs)
While these beasts are not used as mounts by every orc tribe they're nevertheless used as hunting packs by most. They're either to guard the camps or hunt down prey. In battle these packs might fight alongside their masters as wargs can fall into a bloodrush and even chain mail cannot hold off their steel-hard teeth.
Wargs exist in different sizes, from big wolves to nearly horse size. The biggest wargs are being bred by the Rhom-oc and Losh-oc tribe.
Harach-me (Ogre Berserks)
In times of war orcs are known to hunt and capture ogres to use for their war effort. While not very bright ogres nevertheless are immensely strong and can bring down whole dozens of soldiers before collapsing from mortal wounds. On some occasions orcs seem to have made deals with some ogre groups but obviously not even orcs do trust ogres enough to let them walk free within their camps. Thus they prefer to capture them and use them as berserks. Chained, often blinded, and kept hungry the orcs will then unleash these tortured war machines in the front of the battle line and direct them with torches and sticks into the right direction. Half a dozen ogres, even when weakened by long captivity, can still bring havoc to a well-ordered battleline and kill dozens of people before being slain. Still ogres are known to not discern between friend and foe and are thus also a risk to bring to the battle when they turn against the wrong side. So even orcs do not use them on a random basis.