Although Orm and Arthur have always been presented as half-brothers, the circumstances of their relation has changed over the years. In the Silver Age, Orm was born Orm Curry, the son of Aquaman’s human father Tom Curry and a human woman named Mary O’Sullivan. Having grown up in the shadow of his brother Arthur, he held deep resentment for his brother’s remarkable abilities. He turned to crime at an early age, and at one point, lost all his memory when he was stricken with amnesia.
Due perhaps to residual memories and resentments, Orm, now having given himself the surname of Marius, became a high-tech sea pirate. At first, he merely held sea faring vessels for ransom, but eventually moved on to causing natural disasters in the ocean. This brought him into conflict with his long lost brother Arthur, now known as the Super-Hero Aquaman. He eventually created a special helmet that allowed him to breathe underwater, and on several occasions, tried to steal the throne of Atlantis from his brother.
At one point, the heroic spirit who goes by the name Deadman possessed the body of Orm. He unlocked his memories that were stricken from his mind when he initially got amnesia, but access to his full memories only deepened his hatred for his half-brother, and furthered his desire to take his vast kingdom away from him.
After the universe shattering events of the Crisis on Infinite Earths, Orm’s origins were altered along with his brother Aquaman’s. Although he was still Aquaman’s half-brother, the circumstances of that relation were different. In this new reality, neither Orm nor Arthur were the son of Tom Curry, but instead, they were the children of the ancient Atlantean wizard Atlan. (A lighthouse keep named Arthur Curry still raised a young Aquaman and passed his human name on to him). Atlan sired another child with an Inupiat woman, and that child grew up to be Orm.
When a young Aquaman rescued a young Inupiat girl named Kako from a polar bear, this began a romantic relationship between the two. Her cousin, it turns out, was a young Orm, who was instantly jealous of this interloper who had stolen Kako’s heart, and attacked her. Although she didn’t remember who it was who was behind the attack, this incident began the long rivalry between Orin (Aquaman’s Atlantean name) and his brother Orm, even though the two didn’t know of their sibling status at all just yet.
Years later, Orm resurfaced as the Ocean Master, now using magical abilities in an effort to steal the throne of Atlantis for himself. He once even sold his soul to the powerful demon Neron, in exchange for a powerful trident of his own. The catch was, Orm felt intense pain whenever he didn’t have the trident in his possession. With this new power however, Orm managed to conquer a place called the Dreaming City, which was home to an offshoot Atlantean race. It was during this time that Aquaman finally revealed to Orm their brotherly relation, but this realization caused Orm’s mind to snap, not being able to bear the reality that his most hated enemy was also his brother.
Afterwards, Ocean Master joined Lex Luthor’s Injustice Gang for a brief time, coming into conflict with his brother’s teammates, the Justice League. Orm fought the League once again, during a time when it appeared that Aquaman had died, and he tried to claim the kingdom of Atlantis for himself. He even once his magical skills to alter reality, so that Arthur was “Ocean Master,” and Orm was “Aquaman.” As the villainous Aquaman, Orm held the underwater city of Sub Diego under his thrall, but Arthur broke this spell, and reality reasserted itself to what it once was.
Reality in the DC Universe was altered once again, after the time shifting event known as the Flashpoint. In this new reality, the nature of Orm and Arthur’s relation is changed once again. In this new history, Orm was the son of Queen Atlanna, also Arthur’s mother (In previous histories, Orm and Aquaman shared a father instead). After Queen Atlanna has Arthur with a human male, once again Tom Curry, she gives up the child and allows him to raise her young son. Eventually, Atlanna is forced to marry a member of the Atlantean Guard, and Orm is the result of this union.
Twelve years later, Orm’s father dies, and soon, so does Atlanna, under mysterious circumstances. Young Orm is suspected as being behind his parent’s deaths, but with no proof, he ascends to the throne of Atlantis. Knowing that Orm is a corrupt ruler, the Atlantean advisor Vulko seeks out Arthur on the surface world, and tells him that he is the rightful ruler of Atlantis, and must take the throne from his brother. Aquaman allowed Orm to remain King of Atlantis as long as he did not invade the surface world. That was not a promise that was kept.
Due to manufactured circumstances, which made it seem like the humans was looking to destroy Atlantis, Orm invaded the surface world, causing massive amounts of destruction and loss of life. This cause him to finally fight his brother for throne of Atlantis, which his brother wins with the help of his paramour, Mera, and his friends in the Justice League. Orm – now known on the surface world as “Ocean Master” – is remanded to Belle Reve prison. His hatred and jealousy of his brother now ingrained in him for all time, he spends his days plotting how and when he will regain the throne that he sees as rightfully his.