Your family, your lover, all will die by my hand!
~ Heath Oslo to Yuuji Kazami

Heath Oslo is the primary antagonist of the visual novels and anime Grisaia no Meikyu and Grisaia no Rakuen. Heath Oslo is an international terrorist and weapons manufacturer involved in the illegal creation of super-soldiers, and possesses vast resources, in spite of being an internationally wanted man, including heavily armed and highly trained mercenaries, an aircraft carrier and associated military aircraft, and even experimental ultra lightweight, high-yield nuclear warheads. Olso's goals beyond creating and possessing advanced weapons are unspecified, and it is not clear whether he is a terrorist in the traditional sense, with political goals, or simply an illegal arms dealer.

Oslo first meets the protagonist, Yuuji Kazami, when purchasing the part of his child prodigy sister, Kazuki. After the deaths of Yuuji's family, the latter of which, Yuuji's abusive father, was by his own hand, Oslo takes Yuuji in and, bizarrely, dresses him as his sister, though Yuuji later claims he never sexually assaulted him. One of Oslo's associates, however, had no such restraints, and Yuuji was forced to kill in self-defense for a second time- at less than ten years of age.

After seeing Yuuji's potential, Oslo forces him into a "boarding school" in a foreign country, which is actually a training ground for child soldiers- though Yuuji did not know it at the time, this was training ground for super-soldiers. During his time at the camp, Yuuji proves to be a skilled marksman and close combatant. At the same time, he also befriends a girl named Marlin. When Oslo catches wind of this, he cruelly orders Yuuji to fight to the death against Marlin. When Yuuji refused to kill his friend, Marlin brutally beats him.

In spite of his failure to kill his friend, Oslo gives him a second chance and informs him that Marlin died soon after she "graduated" from the training camp. Olso then sends Yuuji on his first of several assassinations, which he would commit around the world, including the US, Canada, and France.

Yuuji is rescued from Oslo's base hidden under his mansion by Asako Kusakabe, a member of a joint US-Japanese counter-terrorism unit, however, Olso escapes the raid.

Oslo is not seen by Yuuji until years later, when Yuuji, now 17, and has faked his age with the help of a friend of Asako's and joined the military, quickly becoming the deadliest sniper in the Japanese military. Oslo launches an attack on a foreign embassy in Tokyo using a clone created from Yuuji's DNA, framing him for the attack resulting in Yuuji's imprisonment. Oslo demands the Japanese government hand over a supercomputer known as Thanatos, as well as Yuuji himself, or he will detonate a nuclear device in Tokyo.

Yuuji is transferred for the trail, only to be attacked by Oslo's men. Against all odds, Yuuji's friends from a year he spent at a high school on leave- in order to live a normal life for a change- Amane Suou, Yumiko Sakaki, Michiru Matsushima, Sachi Komine, and Makina Irisu, manage to rescue him using non-lethal weapons manufactured by Sachi, who has a knowledge of chemistry, as well as Makina Irisu, who was trained in marksmanship by Yuuji. Yuuji's friends are aided by Yuuji's old military buddies during his time training with the US Marines, as well as Thanatos, which is actually directly connected to Yuuji's child prodigy- sister Kazuki, long thought to be dead. It turns out, however, that Oslo placed a timed micro nuclear warhead on Yuuji's wrist in order to control him.

Yuuji's friends rescue Kazuki and escape in a submarine she managed to get a hold of, presumably using Thanatos, however, Yuuji launches a lone attack on Oslo's base of operations, the aircraft carrier, Tartaros, and kills a number of Oslo's guards, including the commander, Travis. Yuuji then engages in combat with his clone and killing him after an extended battle. Finally, Yuuji confronts Oslo, who tells him about his plans to use Yuuji as a prototype for a cloned super-soldier, and that Marlin, his old friend from the training camp, was used as the surrogate mother for the clone and died in childbirth. Yuuji and Olso then engage in combat, Yuuji armed only with a combat knife, and Oslo armed with a katana. Yuuji is severely wounded in the fight, being impaled on Oslo's sword, however, he managed to kill Oslo using by grabbing a pen off Olso's desk and stabbing him in the back of the neck.

In spite of his serious wounds, Yuuji manages to remove the nuclear weapons attached to the bracelet using a key in Oslo's possession and escapes the carrier before it is destroyed by the nuclear warhead. Yuuji is later reunited with his friends, who have escaped to an island in the Pacific.

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