Many UK users ask horse racing to be included in OW. Here is the common answer that become available after certain efforts in math analysis.
Horse racing is a kind of competition that appears to be extremely hard to predict because of the following reasons.
Let's say that you have measured individual performance of the participated horses in terms of 1) math expectation of the running time (mu), and 2) its math variance (sigma^2). That data is a mandatory statistical basis of all further computations.
Let's also assume that running times obey Gauss distribution.
Now the formal solution can be expressed in terms of N-fold semi-infinite integrals of N-dimensional Gauss distribution, where N is the number of horses.
I tried to compute those integrals using the program Mathematica (version 3, 5, etc. doesn't matter). 3-fold integrals took 3 minutes to be computed in 2 GHz system. 4-fold - over 30 minutes with the minimum accuracy. Yet the number of integrals to be computed is equal to N! (factorial). 9! = 362880 9-fold integrals for 9 horses will be computed all the time of the Universe being.
My conclusion is that existing math formalism does not allow to compute desired probabilities/odds in a reasonable time in a horse racing.
To substantiate the answer, I attach the formulae for just 3 horses, where p(ABC) means that A was 1st, B was 2nd, and C was 3rd. Naturally there are 3! = 6 combinations and respective integrals:
Anyway, the work is still continued, and there is a hope to find out some workaround for the above integrals.