Instructions
Play the game until you are finished with all the main story mode quests. Go to the Fossil Guild, situated to the left of the police station, and speak to the info clerk standing in the back. Keep talking to him until he mentions the holes that are appearing in Rivet Ravine. If you do not hear this information from him, the ladder that leads to the secret tunnel won't appear.
Travel to Rivet Ravine and enter the large tunnel located at the upper area of the map. Go down the tunnel and take the third tunnel on your right.
Keep traveling down this tunnel until you reach an area with a four-way split. Continue going up and then right until you reach a ladder in the corner.
Go down the ladder, and you will find yourself in a new tunnel. To find O'mel in the secret tunnel, go left until you reach a sign about the "pure of heart." Take the tunnel marked "O" three times to reach a new tunnel.
Keep walking down this tunnel and ignore the treasure chest once you see it. If you open it, you will be placed back at the start of tunnel, so just keep walking past it. You will eventually reach the final part of the tunnel.
Walk all the way to the back of the tunnel to where the rocks are. You will find O'mel in the secret tunnel.