Ok, sorry if this has already been covered, but, I have looked into this topic a lot! Here is what has happened. A man died in the old safari when he stepped out of his truck to use the... restroom. A lion got him. A man who was playing in a band went to his car to get his tuba (I think) and got struck by lightning. He died.
One day (aka Black Sunday) three people died on the same day. Two died when a man went to splash his friend with water from the lake. The man was electrocuted because of faulty wiring. His friend jumped into save him and died. A security guard mis-heard his radio thinking someone got shot instead of shocked. He jumped in to save the men and died as well.
A woman no less that 15 minutes later fell out of Flight Commander. She was intoxicated and tried to look at the helicopter that was flown in for the three from the pond incident. The seat next to her had not been latched in (she was riding alone) and when she tried to see the helicopter she hit the joystick and flipped the car. She slid into the next seat and fell out.
A woman on Top Gun (Flight Deck) had a heart attack and later died in the hospital. A man on Firehawk had breathing problems after getting off. He also later died in the hospital.
Lastly, and probably the most famous, was Tower Johnny. On graduation night, a man climbed to the 50 foot level of the Eiffel Tower. He was very drunk. He was going to try to jump onto the elevator and scare his friends, but instead he got hit by the counterweight and was cut into pieces by the cables holding the tower up.
Those are all that I know of that happened while the land was Kings Island. I know of a few that have happened before it was an amusement park. Let me know if you want to hear them.