In a nutshell, O1 steel is easy to hone and a very fine edge achieved relatively quickly. PM-V11 alloy blades that are easier to sharpen than and hold an extremely durable cutting edge for longer periods between sharpening. What does this mean in practice? If you’re roughing something to size or planing abrasive exotics then PM-V11 is probably the better choice. However, for fine finishing or the pure pleasure of working with the very sharpest of tools then go for the O1 option. These 3.6mm(9/64") thick x 35mm(1.3/8") wide lapped blades are suitable for Veritas NX60 and DX60 block planes.