I have a 1988 Mustang GT 5.0 that will be back on the road within the next month or two. It hasn't been started in at least 5-6 months. Will this have any affect on the oil pressure being that it hasn't been started for awhile? Other than changing the oil and the spark plugs, what other preparations should be done.  

Yeah your best bet would be to change the oil, check the plugs , cap rotor & wires- might be white chaulk on them.  Ive heard a lot of old timers put a squirt of penetrate oil in each cylinder before installing the plugs to break up carbon/varnish in the rings.  if the gas smells bad replace, check/test coolant & add anti corrosion additive ( I get penray from carquest) you might as well clean/ replace air filter, pcv valve & fuel filter. A good top engine cleaner like sea foam  or if you can find if GM top engine cleaner (dealer or some older auto parts stores) & suck into intake through large vac hose.  scrub the throttle body & spray inside the mass air meter (if equipped), IAC valve & for good measure put a can of fuel line anti freeze to get rid of the condensation that may be laying on the bottom of your tank now.  charge the battery & have fun.  Check the seals after a good warm up drive as they usually sprout leaks after sittin around.  good luck  

If it's only been 5-6 months since the motor last ran, change the oil and use 5W-30 so it'll build pressure fast, and get some Marvel Mystery Oil and put a little bit into each of the cylinders for good measure.

Your gas should be ok after only 5-6 months of sitting.