What does an 'O' mean in the DOT hazardous materials segregation table under 49 CFR 177.848?

49 CFR 177.848(e)

Reading 'O' as 'no restriction' is a dangerous mistake in the field — it would lead a driver or loader to skip a leak-containment separation step the regulation actually requires. The real meaning means two materials marked 'O' still need protection against commingling if something leaks, not a free pass to load them side by side with zero planning.