You must find a way to tell Og how to play Rock, Paper, Scissors. But how do you show those objects if they haven’t even been invented? Simple: invent them!
Pick up the ROCK and the STICK in the cave. Then use the stick on the rock to create a HAMMER. Leave the cave and find a piece of ORE. Walk to the north to the river. There are reeds along the shore. You can take the third REEDS from the left.
Walk to the left and enter the cave again. Use the ore on the fire to melt it into METAL. Use the hammer on the metal to create the SCISSORS. Use the hammer on the reeds to create PAPER.
Use Og again to play Rock, Paper, Scissors.
Game source: A copy of the game was found here on the internet.