Throughout the episodes, there are many hints to Oh Il-nam being the creator of the games.
His number in the game was 001; as the creator of the games, he was naturally the first to have the opportunity to enter himself.
From Episode 1:
Il-nam is an outlier among the players. He is elderly, while the others range from young adulthood to middle age. The others are there because they are in dire financial straits, and stay because of the possibility of winning a huge amount of money. Il-nam has a brain tumor, little time left, and no financial motive to compete. In the next episode, he tells Gi-hun that he's going back into the contest because he doesn't want to wait around to die, but that doesn't explain why he would have been chosen in the first place.
He's the first to resume playing following the massacre, which on first viewing might be interpreted as him thinking "I'm dying anyway, I might as well have some fun". In hindsight, however, it seems he wasn't shocked into paralysis like everyone else as he already knew the penalty for losing the game was death, and must have witnessed this kind of thing many times before. Furthermore, when we see everything from the doll's point of view, the camera scans him a split second later than the rest and with a lighter shade of green, implying the rules are much more lenient for him.
Episode 2: We see what each of the major contestants' lives were like back home... But the most we get from Il-nam is him confiding in Gi-hun that he wants to continue playing despite being the deciding vote to end the games.
When asked by Gi-hun if he also lives nearby when they encounter each other in the outside world, Il-nam states he's staying at a friend's place and otherwise has nowhere else to go. This is vague enough to suggest he might be homeless, but still seems unlikely since in a financial situation like that, how likely would it be that he could access medical help in even recognizing his brain tumor?
Episode 3: Il-nam's completed honeycomb shape has a few hairline cracks at the edge. Due to contestants being shot by the trigger-happy Soldiers over the smallest mistakes, Il-nam surviving despite his shape technically being imperfect hints towards him being more important than we might assume.
A subtle one in Episode 4: the Front Man immediately stops the infighting within the player quarters when Il-nam starts rambling on the violence.
Another subtle one is in Episode 5, where you can clearly see his handcuffs aren't locked onto the rope, while everyone else's are.
In Episode 6:
During the Marbles game, when he lets Gi-hun know he was aware the whole time of him he trying to use his dementia to trick all the marbles out of him. He asks Gi-hun an Armor-Piercing Question about fair play with a partner. The Front Man always claimed the games are to be completely fair across the board, and he takes orders from Il-nam. Il-nam was subtly warning Gi-hun that he was starting to become like the selfish humans that he hates.
It seems a bit odd that Il-nam's death wasn't actually shown for a series that rarely shied away from the violence, and the episode had no problem showing Ji-yeong's similar Heroic Sacrifice earlier. Likewise, Ali's dead body is seen at the beginning of the following episode, but Il-nam's body is never shown.
There's no profile for Il-nam when Hwang Jun-ho checks the files in the Front Master's archives.
In addition, he's a little too carefree when playing some of the games, particularly in Marbles and Red Light, Green Light. In the former game, the guards aren't too quick to kill him once it's established he's lost, even if it's an act.
There are basic pictograms of all of the games on the walls in the players' room, arranged in the order that they'll be played in. Nobody notices these details, but they become more apparent after most of the player bunk beds have been removed.
In Episode 7, Gi-hun hesitates before stepping onto the glass panes, and there's a shot from his point of view where he looks from one pane to the other. In that shot, the lights in the room are reflected in the glass, which is how Player 017 recognizes the tempered glass.
During the second game in Episode 3, we hear the announcer mentioned that Player 69 has passed the game, followed closely by Player 70. We're later revealed that both of them are a husband and a wife in the next episode, which explains why both of them finished the second round together in a quick succession.
In Episode 2, some characters have a scene that will later be evoked by the way they die.
Ali — Steals money from his boss in front of him to compensate for his delayed paychecks. Dies in the marble game after Sang-woo secretively steals his marbles in front of him.
Deok-su — Kills a henchman who betrayed him and jumps off a bridge to escape from some oncoming Filipino gangsters. Dies in the bridge game when Mi-nyeo, who became his henchwoman during the games but was betrayed by him, falls to her death with him in her grasp.
Sae-byeok — Threatens to kill a smuggler by pointing a knife at his neck. Dies in her bed, before the final game, after being stabbed by Sang-woo in the exact same spot.
Sang-woo — Tries to commit suicide while drenched in water inside a bathtub, but is interrupted. Dies in the final game by killing himself with a stab in the neck (the same way he killed Sae-byeok and also in the exact same spot) while drenched in rain.
When Mi-nyeo applauds her team for winning the tug-of-war game, she says that she felt at her most powerful when she was leaning backwards as part of Il-nam's strategy. She would use the same method to kill Deok-su and herself in the fifth game.
When his daughter's stepfather tries to give Gi-hun the money for his mother's medical treatment just so long as he agrees to stay out of his daughter's life from now on, Gi-hun lashes out at him, saying "You think money solves everything?!" Gi-hun ends up the Sole Survivor of his game, but is far more miserable and despairing than he was when he started and actively despises the fortune he has won.
Those with knowledge about Korean literature could probably have foreseen Gi-hun returning home in Episode 9 with the prize money, only to find his mother had died in the meantime. The title of the episode is "One Lucky Day", the same as a famous Korean short story where a rickshaw porter, the ancient equivalent of a chauffeur, has a lucky day where he gets a lot of customers and earns lots of money, only to return home to find his sickly wife dead.
In episode 5, Ji-yeong mocks Player 244's Christian faith by sarcastically praying to God to send the other team to "his place." In episode 6, it's revealed that Ji-yeong's father was a pastor who was abusive to her and her mother, and ended up killing the latter while justifying the act through his faith to God.
