The k55k is a better carbon steel than an Opi I think. It is also more standard slipjoint construction. It is also about twice the price.
Also, the tip hits a plastic piece inside the the knife to stop closure instead of the kick at the base of the blade hitting. The tip hits in an Opi also but without a spring it doesn't dull the tip as much. I like the Mercator quite a bit otherwise but this is a fatal flaw.
The Svord has great steel, but it is essentially a friction folder kit that you can make a nice knife out of.
Out of the box the Opi wins I guess. If you like to tinker and can deal with the awkward carry the Svord might win.