Orontes II was the son of Orontes I

Orontes I

Orontes I Armenian King of Orontid Dynasty reigning in the period between 401 BC – 344 BC. The Persian version of the name is Auruand which meant "Great Warrior" in the Avestan language....

.
After Codomannus

Darius III of Persia

Darius III , also known by his given name of Codomannus, was the last king of the Achaemenid Empire of Persia from 336 BC to 330 BC....

 ascended the throne of Persia as Darius III in 336 BC Orontes was given the Satrapy of Armenia

Satrapy of Armenia

The Satrapy of Armenia , also known as Orontid Armenia after the ruling Orontid Dynasty, was one of the satrapies of the Achaemenid Empire in the 6th century BC, which later became an independent kingdom...

 to rule.

At the Battle of Gaugamela

Battle of Gaugamela

The Battle of Gaugamela took place in 331 BC between Alexander the Great and Darius III of Persia. The battle, which is also called the Battle of Arbela, resulted in a massive victory for the ancient Macedonians and led to the fall of the Achaemenid Empire.-Location:Darius chose a flat, open plain...

 he fought on the right flank for king Darius III, with 40,000 infantry and 7,000 cavalry under his command. He died there.

Ironically his son, Mithrenes

Mithrenes

Mithrenes was an Armenian commander of the Persian force which garrisoned the citadel of Sardis....

, Satrap of Lydia

Lydia

Lydia was an Iron Age kingdom of western Asia Minor located generally east of ancient Ionia in the modern Turkish provinces of Manisa and inland İzmir. Its population spoke an Anatolian language known as Lydian....

, had joined king Alexander III after being defeated at Sardis

Sardis

Sardis or Sardes was an ancient city at the location of modern Sart in Turkey's Manisa Province...

in 334 BC. He then fought at the Battle of Gaugamela on the side of Alexander.
After the battle he was made Satrap of Armenia by Alexander.