I'm unclear about what you are doing exactly when you say oversampling with inverse priors. If you are using the Sample node to sample a higher proportion of rare events, then you would need a Decisions node following it to adjust the prior probabilities. When using the same prior probabilities, it is valid to compare the models with different event proportions from oversampling. The "Prior Probabilities" section of the same part of the EM Reference Help that I mentioned above explains this better than I can!
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