So I can use the SSI-4 to monitor oil pressure, but to do this, Can I simply tap into the OEM oil pressure sensor line? Or do i need to buy a separate oil pressure device?

You would use the stock location

I dont know what an SSI-4 is, but if uses an electrical signal you will need to get a pressure sender. 4th Gen maximas don't have a pressure sender, they have an on/off switch for the idiot light. Place a T fitting in the stock location and run the stock switch along with a pressure sender.