"fuel pressure at idle is 39.5 and increases and decreases with load and vacuum"
While 39.5 is a good number what are the max and min you are seeing? Should be steady at about 40 psi under all loads and rpm.
I would take a look at the Fuel Pump Driver Module (FPDM) as these are known to corrode away on the 2004-2008 F150s, hundreds of posts.
Check the condition of the ground wires next to the fuse box by passengers kick panel, hundreds of posts.

BTW my comment about the O-ring on the dipstick was meant to highlight there are a lot of subtle causes for the P2195

Post back as you progress towards the solution

 

The code is for an excessively Lean fueling from an air leak or lack of fuel.
The OX Sensor is just detecting it.
It's not the Sensor.
Find the Air leak in the intake system or the exhaust ahead of the Sensor.
Its unaccounted-for Air that did not get accounted for by passing through the Mass Air meter.
Good luck.