I'm still having trouble establishing a telepathic link to your truck. What size tire are you currently running? The commonly accepted maximum size tire you can fit on the OBS truck is ~33 inches, which allows you a nominal 6-6.5 inches of sidewall with 20 inch rims. RatRods proposed 275/65/20's would likely be a little too big without lifting. If your currently running a nominal 29-31 inch tire, then you have some room to increase tire size without lifting. If you're already running a 32-33 inch tire, then you'll either have to lift to fit a bigger tire, or you'll have to go with a smaller rim to get more sidewall.
If you don't know how to interpret metric tire sizes, there are several calculators on the internet, or build your own.
1st number=nominal tread width in mm
2nd number=aspect ratio as percent=sidewall height/tread width
3rd number=rim diameter in inches.
Sample calculation
OD=TW*AR/100*2/25.4+ID Where
OD is overall tire diameter
TW is tread width
AR is aspect ratio
ID is rim diameter (tire inner diameter)
275*65/100*2/25.4+20=34.1 inches.