User blog comment:Nobody700/How different is oars?/@comment-1595565-20140108185617

The symbol on the Punk Hazard gate appears to be an OP equivalent to the skull and cross bones seen in the real world. My guess is Oda uses a different symbol to denote danger so as not to mislead people into thinking it was immediately pirate-related. Nami had the same symbol on the outfit she was wearing in the 693 color spread. We don't see too many danger signs or toxic chemical warnings in OP so I'm not surprised we haven't seen it before.