Make sure you have labeled clearly the group for which you want to calculate the oversampling rate. If the data are continuous (such as height or weight), set a cutoff point instead, and call all data within that cutoff area your group.
Calculate the proportion of that group in your sample data. To do this, divide the number of data points belonging to that group by the total number of data points. The result will be a number from 0 to 1.
Find the proportion of that group in the population for which you wish to generalize the data in the same way. You may need to gather some demographic statistics first if you are not familiar with the population. This proportion will also be a number between 0 and 1.
Divide the sample proportion of that group by the population proportion of that group. This number is the oversampling rate.