The leaking oil is coming from the corner of the cylinder head and not the empty threaded hole in your pic. Either the valve cover gasket is leaking at the corner OR the #5 cam bridge/cap has been removed at some point and not properly resealed when reinstalled. Messy... yes. A Danger... nope.

 

 

The bolt that goes in that hole originally held a bracket in place... a bracket that located the OE air box.

 

 

OP, did you read this above ?!? The hole is a "Blind" hole... meaning it is not originally drilled into any fluid passage... oil or coolant. So unless someone has run a bolt that is WAY too long into the hole and cracked the head, it is NOT leaking. Spray off the end of the engine that you have pictured with brake cleaner really well and then take the car for a test drive. Then re-inspect the location pictured. The leak will come from the highest point that is wet... and it may be either closest to the front or rear of the engine depending on the angle (lean) of the engine and the under hood air flow.