? oil leak-block to head at oil pressure sensory gallery?

It is on the engine somewhere. Do a Google search for oil pressure sender location and include the vehicle year, model, and engine size.

Ether the oil pick up screen is plug up or your oil pump gave up.

The only place, oil and water share a gasket is the cylinder head gasket, oil and water galleries run through the engine block and head via the head gasket. The cylinder pressure will be OK, it's the oil pressure that's putting the oil in the coolant, the gasket has gone between the water and oil galleries or there is an internal crack (unlikely). A coolant pressure test should show it up.

a low oil pressure, have the oil pump pressure tested

Sounds like a blown cylinder head gasket between the water and oil passages and galleries. Is the amount of oil significant..not just a hint of a film? Do you have any sign of creamy looking 'mayonnaise' on the under side of the oil filler cap and entrance? This would be sign of the reverse i.e. that water had got into the oil. The oil in the engine is driven around the galleries under pressure, certainly more pressure than the coolant, so if there is a connecting leak oil will be forced through it into the coolant. The best way to make sure is to have your coolant system pressure checked by you local garage when the engine is really cold. With the engine off and the oil filler cap removed a leak will be indicated by the coolant pressure dropping slowly (as water is forced through the leak back into the oil system). Also monitoring the pressure of the coolant when the engine is started from cold will also reveal a leak. The coolant is at normal atmospheric pressure when cold and only when the engine gets hot does the pressure within the coolant system rise (somewhere between 7 - 10psi depending on power unit). However if there is a leak the pressure of the coolant will rise rapidly as the oil system reaches pressure quickly, in other words the coolant system will be forced to the same pressure as the oil system through the leak. The remedy is to renew the head gasket.

Replace faulty oil pressure sender.
Oil pressure sender is usually on the engine block, screwed into a fitting accessing the motor oil distribution gallery.

where is oil pressure switch/sensor on 2001 nubira

Still can be a cracked head gasket. Unless you are creating to much pressure in your engine. Just because you do not see any oil leaks does not mean that the head gasket could not be cracked

Here is a link to what you need and the guide below

http://i253.photobucket.com/albums/hh43/chuck943/A14937D.gif

Item Part Number Description 1 — Valve Tappet Vent Gallery (Open to Crankcase) (Part of 6049) 2 — Valve Tappet and Camshaft Bearing Feed Gallery (Reduced Pressure) (Part of 6049) 3 — Orifice and Strainer (Part of 6049) 4 — RH Head Gallery Oil Reservoir (Part of 6049) 5 — Primary Main Gallery Pressure (Not Restricted) (Part of 6010) 6 — LH Head Gallery Oil Reservoir (Part of 6049) 7 — Orifice and Strainer (Part of 6049) 8 6049 Cylinder Head 9 — Oil Drain to Sump (Part of 6049) 10 — To Hydraulic Timing Chain Tensioner (6L266) 11 — From Left Bank Cylinder Block (Part of 6010) 12 6714 Oil Bypass Filter 13 9278 Oil Pressure Sensor 14 6881 Oil Filter Adapter 15 — Oil Pump Integral Relief Valve (Part of 6010) 16 — Bypass (Part of 6010) 17 6600 Oil Pump 18 — To LH Cylinder Head (Part of 6049) 19 — Oil Drain to Sump (Part of 6010)

there are oil galleries in the had so that the valves and cam can get some lubricant so it maybe that in one of those oil galleries has a crack and same with the head gasket so it could be either one