Oslo「オスロー」 is King's pet Black Hound, an insanely vicious wild creature that will hunt enemies until it successfully kills them. It is later revealed that Oslo is the reincarnation of Rou from 3000 years in the past.
Oslo is a dog-like creature with green fur, who is covered in scars, with his neck being the only area with white fur. Oslo has a fairly short tail, sharp and piercing teeth, and grows larger with it feels threatened, as all black hounds so. It shrinks into a small dog when not feeling threatened at all, with its scars not being visible when it is in the form of a small dog.
Oslo, just like all black hounds, is an insanely vicious that will not give up on a prey until it successfully kills it. However, Oslo appears to be quite submissive toward King, and acted like a normal puppy before them.
Oslo was born 3000 years earlier after Gloxinia mysteriously disapeared, protecting Gerheade in previous Fairy King's place. How Oslo met King and why he is submissive towards him has not been revealed yet, but it was revealed that Oslo knew King many years before he met the Seven Deadly Sins, in the Fairy Realm.
Capital of the Dead arc (manga only)
Oslo was attacking and killed a Holy Knight apprentice Andre and his partner who were about to attack the Sins. Oslo was about to attack them but Ban threaten the beast but he grew larger due to his ability but Meliodas used his sword to created a illusion of a very huge, fearsome, ethereal dragon-like creature which scared Oslo who shrunk down and ran away. Oslo later returned back to King his owner and after King's first fight with Ban, King returned embarrassed because Diane was there and he said she was so cute and told Oslo what to do but King and Oslo saw the Sins and they were trying to find a way to the Capital of the Dead and then a bunch of flowers were going wild and they disappeared along with King. For now Oslo is waiting for King to come back.
King later summoned Oslo through a portal that the latter made and introduced it to Diane and revealed Oslo's ability to teleport and the fact he never bought him to Boar Hat was because Oslo loves pork. King told Oslo to teleport Diane to Liones but she was too big and King ordered Oslo to get bigger so he didn't have to push Diane's butt. As Diane was being attacked by the Holy Knights, Oslo felt Gilthunder's Thunderbolt and ended up getting damaged. King went to Oslo to see if he was all right, but manage to sent King to Liones to save Diane.
Post-Kingdom Infiltration arc
Oslo later appeared in Liones while King was debating whether to ask Diane out on the festival or not due to her being possibly depressed over Meliodas and Elizabeth's relationship.
Great Fight Festival arc
Alongside his master, Oslo and King meet with the revived Elaine and soon rejoin Meliodas in the Vaizel Great Fight Festival. Oslo being paired by Matrona, a giantess and Diane's old mentor.
Oslo is very quick and powerful, which is seen when he quickly defeats two Holy Knight apprentices with ease. He also has the the ability to change his size depending on how threatened he feels. He grew very huge when Ban released a lot of killing intent toward him, and was later seen to be as small as a small dog when not threatened at all. Like all Black Hounds, Oslo have the unique ability which allows it to send anything it swallows to a completely different location.
Seven Deadly Sins
King is Oslo's master and is very loyal to him.
At first, Oslo sees Hawk as food and someone to mess around with, aka, eating his meals. However, after training with him at Istar and defeating a Tyrant Dragon by eating its inside, Oslo grow to respect Hawk, calling him big bro or captain, somewhat joining his Knights of Leftovers.
- Meliodas vs. Oslo: Loss
- Oslo favorite food is pork, which is the reason why he was never introduced to everyone in the Boar Hat.
- In the Nanatsu no Taizai Popularity Poll, Oslo was 21st place (21 votes) for postcards, but 6th place (4285 votes) including online votes.