The historical origins of the Ancient Olympic Games are unknown, but several legends and myths have survived. One of these involved Pelops, king of Olympia and eponymous hero of the Peloponnesus, to whom offerings were made during the games. The Christian Clement of Alexandria asserted, "[The] Olympian games are nothing else than the funeral sacrifices of Pelops."[2] That myth tells of how Pelops' overcame the King and won the hand of his daughter Hippodamia with the help of Poseidon, his old lover, a myth linked to the later fall of the house of Atreus and the sufferings of Oedipus.
Another myth tells of the hero Herakles, or Heracles, who won a race at Olympia and then decreed that the race should be re-enacted every four years, while another claims that Zeus had instated the festival after his defeat of the Titan Cronus. Yet another tells of King Iphitos of Elis, who consulted the Pythia Oracle at Delphi – to try and save his people from war in the 9th century BC. The prophetess advised him to organize games in honour of the gods. The Spartan adversary of Iphitos then decided to stop fighting during these games, which were called Olympic, after the sanctuary of Olympia where they were held. Had they been named after Mount Olympus, the mountain on which the Greek gods were said to live, they would have been called Olympian games rather than Olympic. The favorite story is that Heracles celebrated cleaning the Augean Stables by building Olympia with help from Athena.
Whatever the origin, the games were held to be one of the two central rituals in Ancient Greece, the other being the Eleusinian Mysteries.[3]
Another possibility for the actual origin of the Games is that they essentially 'evolved' from Funeral Games.

Unlike the Modern Olympic Games, only free men who spoke Greek were allowed to participate in the Ancient Games. They were to some extent "international", though, in the sense that they included athletes from the various Greek city-states. Additionally, participants eventually came from Greek colonies as well, extending the range of the games to far shores of the Mediterranean and of the Black Sea.
In order to be in the games one had to qualify and the athlete had to have one's name written down in the lists. It seems that only young people were allowed to participate, as the Greek writer Plutarch relates that one young man was rejected for seeming too mature, and only after his lover interceded with the king of Sparta, who presumably vouched for his youth, was he permitted to participate. Before being able to participate, every participant had to take an oath in front of the statue of Zeus saying that he had been in training for 10 months.

Athletes running the hoplitodromos
The Olympic games originally contained one event: the stadion (or "stade") race, a short sprint measuring between 180 and 240 metres, or the length of the stadium. The actual length of the race is unknown, since tracks found at archeological sites, as well as literary evidence, provide conflicting answers. Runners had to pass five stakes that divided the lanes: one stake at the start, another at the finish, and three stakes in-between. Since time was not pertinent to winning the stadion, merely passing the finish stake first was enough to earn the victory.

The Ancient Olympic Games, originally referred to as simply the Olympic Games (Greek: Ολυμπιακοί Αγώνες; Olympiakoi Agones) were a series of athletic competitions held between various city-states of Ancient Greece. They began in 776 BC in Olympia, Greece, and celebrated until 393 AD.[1] The prizes were olive wreaths, palm branches and woollen ribbons.