I needed to pry open some heavy duty S-hooks to change out a playground swing. There were different suggestions online, but none that seemed like they would work for S-hooks this thick.
Finally I realized that I could hammer a chisel into the gap, and it worked!
Now I just need a better way to close them back up — I held them with a Vise-Grip and hammered on them, but they rotated some and got sharp barbs where the Vise-Grip tore them. I had to grind them down with an angle grinder.