DIY metal roofing question?

They make nail and screws with sealing washers. I have used the corrugated panels right on the joists. Probably not with 24" OC though. 16 works. Don't really need the OSB but as long as you have it, why not? 

https://www.homedepot.com/pep/Everbilt-10-x-1-in-Clear-Head-Roofing-Screw-1-lb-Box-125-Piece-116156/310014231?source=shoppingads&locale=en-US&pla=&mtc=SHOPPING-CM-CML-GGL-D25H-025_003_FASTENERS-NA-MULTI-NA-PMAX-7127963-NA-NA-NA-NBR-NA-NA-NA-Fasteners&cm_mmc=SHOPPING-CM-CML-GGL-D25H-025_003_FASTENERS-NA-MULTI-NA-PMAX-7127963-NA-NA-NA-NBR-NA-NA-NA-Fasteners-20499493109--&gclsrc=aw.ds&gad_source=1&gad_campaignid=20278983267&gclid=EAIaIQobChMI_tm027vekAMVlJbuAR1seDmiEAQYAiABEgJ_BfD_BwE

I use those exact screws and they work great right into half inch OSB, no need to hit joists. 

If you are in Florida you might want those furring strips to give your OSB a little more ventilation during the (12) humid months of the year. At any rate just make sure you put the screws in the peaks not the valleys.

Standing seam or just corrugated sheets?

Do you have a FL approval number? If so look it up and it will tell you how to install. I have never seen and I would never screw to the osb. Add the firing strips.

I think I would add strapping (furring). Nailed right into the joists. 1/2" OSB isn't very confidence inspiring for screw holding power.

If you put furring strips, you have to think about your trim pieces.

Maybe block it out under the sheet stock where you are going to put the cleats, vapor barrier on top off the plywood.

If you have OSB sheathing, no need to add furring straps.

Furring strips would be used if you didn't have sheathing. No need for 2 layers of sheathing.

If you have standing seem roofing, you have the good stuff. 

You would trust 1/2" OSB to hold roofing screws?

That's how every roofing material is fastened, through the sheathing.

Go on you're roof and grab a shingle and rip it off. 99 percent of the time the nails stay in the sheathing. 1/2" OSB is pretty standard sheathing now a days and I can guarantee the op won't have any issues. It passes building code.

I know just a little about this stuff.