Pick Earliest Date For A Record From Row Of Those Records?

We have multiple work orders per opportunity, and each work order has a date. I have a sheet of these work orders which shows the work order #, work order date, and opportunity date. What I want to do is add a column which reports the work order date if it is the earliest instance of a work order for the opportunity (said differently, the first work order for the opportunity). Is there a way to write a formula to report whether the date of the row is the earliest date for a given opportunity, all in the same table?