For a metal roof would 7/16" OSB roof sheathing be enough holding power? It's only 1/16th thinner than 1/2" OSB so it shouldn't be that much of a difference in strength or would it?

My roof is 5/8" OSB. There's not that much difference in cost considering the total cost of a house, but it is a lot of work getting it up there. I was working alone, and dragging 72 sheets at 66 lbs. each up a ladder is work indeed. Even though I am in a fairly low wind area, I had my trusses beefed up, and nailed the sheathing to FL standards- 4" spacing at the edges, and 8" field. I also tied it down with the biggest clips they make. I guess I'm paranoid after living in hurricane country for 30 years.

As for the 7/16"- What does the manufacturer recommend? You don't want to over drive the screws anyway, as the gasket under the screw head wants to be compressed just enough; not too much. I would think that enough wind to tear off the roofing would tear off the OSB as well. The roof on my shed is 7/16, and the screws seemed to have plenty of bite.

I went with 5/8" OSB roof decking also. This is with #30 roofing felt and architectural shingles. The 7/16" OSB just seems so flimsy when installed on 2' center trusses. But my framing crew was cussing me before it was all said and done due to the extra weight of the 5/8" OSB. My framing crew used Senco staples to attach the roof decking to the trusses. If I hadn't used 5/8" OSB, I would have used 1/2" or 5/8" plywood. 1/2" OSB isn't even available in my area.

This is covered by the manufacturer and code. Ask manufacturer what they will warrantee. Your location and wind zone will govern the fastener schedule When wind takes off a metal roof it peels it off. Once that starts all the winds force is concentrated on a few fasteners at a time like a zipper opening one tooth at a time. The trick is to not let that process start. I would not be comfortable with 7/16 OSB on any roof in any area. I'm in a 120 mph wind zone and used 5/8 plywood and doubled fasteners at the edge zones and tripled them at the corners - but I've also been called obsessive.... My roof is a standing seam with all concealed fasteners - ribbed roofing with watered fasteners poses a trade off between too few fasteners and too many penetrations. So short answer is - it depends. Ask the manufacturer.

Is this for something existing or something you have yet to sheath?  When I was looking into a steel roof, some of the mfg called for 5/8"; but I don't know that a company would refuse to install it if you have an existing structure with 7/16".  The old part of my house is 1x6 boards anyway.  I used 5/8" for the additions I built myself.  With the trusses 24"OC, it was worth the relatively small price increase for a flatter, sturdier roof surface.  Just my bias.