I have worked O-1 for many years now and have learned to deal with its many quirks. First of all, a spheroidizing treatment has been suggested and that is a good idea, it is your annealing practices that are causing quite a bit of your problem. With steels over .8% carbon heating above critical and then allowing a very slow cool gives any carbon over .8% a chance to gather up and get into trouble, it will sheet up and makes drilling impossible but even worse it will also gather in the grain boundaries and weaken the steel toward the brittle side no matter how soft you may want it.
If the cracks you noticed were like a surface crazing running along the length of the spine almost appearing as if the inside swelled and cracked the outside, I know that issue very well and it is rather common with O-1. This is a result of overheating and allowing to cool all the way to room temp from this overheated condition. O-1 is a deep hardening steel, if you enlarge the grains and increase its hardenability even more, if you cool without cycling to refine the grain the outer surface will harden so differentially from the inner mass that it will be like a skin splitting over an expanding core, this is why it will not happen at the edge but be worse in the thicker areas.
O-1 must be stepped down from any heats above 1600F, just cool until the magnet begins to stick at around 750F-800F and then reheat until it just loses magnetism again. Just one cycle can save you a lot of heartache where those cracks are concerned. As for the anneal, forget all the forge cooling, wood ashes, vermiculite etc... save them for steels with lower carbon contents. After forging, cycle down and then reheat for multiple cycles in the range of red but still magnetic (around 1300F), or, if you have a rampable kiln, soak for 45 minutes at 1375F and then cool no faster than 50F per hour. The first should get you little spheroidal carbides that will give you no trouble, but may still be a bit tough at times, the second will definitely spheroidize the stuff enough to do any machining that you like.