I keep losing oil pressure in my 2004 f150 with a 4.6 litre. I have changed the filter and oil. What is the chance that the oil pump is bad? If so is it located inside the oil pan? Can I change the pump without pulling the engine

You either have a bad oil pump, too much clearance in engine components or a bad oil sensor.

The first thing I would do is chek the oil level.

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Worn main and rod bearings on the crank shaft. This is what creates the oil pressure. When the get worn they start loosing pressure. Also when the oil gets hot, it gets thinner, causing more lose of pressure.

Did you change the oil filter? Make sure you use the large one for the TDi. I had the same problem after replacing my oil pump and cleaning the pick up screen. Turns out it was just the synthetic oil washing the gunk out of the motor and quickly clogging the oil filter. You can verify the oil pump output by connecting an after market oil pressure gauge to the forward left top corner of the oil cooler/filter mount where you will find a removable screw giving you access to a port leading directly to the oil pump. It is awkward to remove then install a line to a pressure gauge but be glad it is at least there so you can verify output pressure. The fitting on the rearward side of the oil cooler/filter mount that has the turbo oil supply line is a good place to verify engine oil pressure with a gauge.
Change your filter first then do what I did and cut the old filter open to see all the junk that may be clogging your filter.
Also, do yourself and us a favor by using spell checker and proof read before posting.

Have you checked if the fault is your oil filter,probably the filter pressure valve inside doesn't keep any pressure? or the other thing is a possibility of the pressure pipes are clogged or semi clogged.

A flashing oil light usally means low oil pressure. If the oil pressure drops below a set point the light will contiune to flash even if the pressure rises back up to normal until you resart the car. Start simple and check the oil. If low then add. Keep an eye on it, if your losing it, its going somewhere, exhaust, or out a seal or if you went to jiffy lube, probably out of the oil filter due to it not being tightened. If oil is ok, your oil pump may be bad. That job is a little complex.

1st you need to do an oil pressure test. If it reads between 20psi-60psi, then it's Ok and you have a bad oil pressure switch. If it's below the above specs then you have an internal engine problem - IE: bad oil pump, bad bearing, etc...

Several issues could exist:
Faulty light - get computer code reading no code good chance its a light/short
Oil pump low pressure test with external pressure meter
Oil pressure sending unit

Once you determine you have oil pressure is good then its a fair chance you have a bad sending unit otherwise its a bad oil pump. Replacing an oil pump is not hard if you ca get the pan off without lifting the motor, Otherwise this is a big job

check oil pressure 

replace the oil pressure sensor