the previous owner disconnected it. and i have no clue on switching it back on. please help?

 

The stock gauge connects to a sending unit located below the oil filter pedestal.
There are two senders in this area...the larger of the two is the oil pressure and the smaller is H2O.

If the PO just disconnected the unit there should be a single wire hanging loose in the vicinity.
If he was more creative in his vandalism, all bets are off.