I believe you mean objective way to define simplest. The problem is that in this particular situation, all definitions of simple are subjective, meaning they rely on the opinions of the person invoking Occam's Razor.
Creationists are perfectly valid in saying that their view draws on the least number of assumptions and hypotheses, which makes it the simplest explanation. They generally cite the Bible as their source.
Evolutionists are also perfectly valid in saying that their view draws on the least number of assumptions and hypotheses, making it the simplest explanation. They cite science as their source.
In this particular case, I do think that using Occam's Razor must favor evolution. The reason for that is that there is solid scientific evidence pointing to the Earth being millions of years old. We have fossil records, skeletons, and examples of evolution at work which are facts. Creationism, on the other hand, requires a healthy dose of faith. Faith is, by definition, belief in something which can neither be proven nor observed. To my mind, that introduces more hypotheses and assumptions, which makes it less likely in the context of Occam and his idea.
I have often wished that he and evolution theory had co-existed, so he could address the issue himself. Being a religious man, I am sure he would have done so.
I cannot give you a simple answer, though I wish I could. I can only tell you two things which people have related to me.
The first is one that a professor mentioned to us in a class in college. We were discussing Occam's Razor and logic, and he said, "If you hear hoofbeats, think horses instead of zebras, UNLESS you happen to be standing near a watering hole in Africa, and there are large predators, as well as large herds of game animals around." His point is that you have to use logic and common sense, as well, when reducing something to its simplest.
The other is something said to me by my nephew. He happens to be very religious, but his take on the whole thing is this. People call it the "theory" of evolution. There is too much scientific data for it to still be called a theory, in his opinion (an opinion I happen to share). As such, evolution is what should be taught in science classes, as there is hard science to back it up. What should be called a theory is creationism. He says that it has no scientifc basis, and no factual data to back it up, and as such, it should not be taught in a science classroom--it should be taught in philosophy classes, as it is more appropriate in that setting.
Both observations seem very wise and logical to me. They are both, in my opinion, elegantly simple. I don't even need to consider Occam's Razor to reach my conclusion.