Make an Opportunity text field required based on a value selected in a picklist value.

Hello,We have a field on Opportunities that we use to track the main competitor.  It is a pick-list and we run into situations where the competitor is not in the pick-list.  We have a value of "Other" in the pick-list.  When our users select this value I would like to make a text field called "Competitor Other Details" required.   The reason is we need them to enter the competitor and it needs to be a text field because we may not know of the competitor, and can not add all possible competitors to the Competitor pick-list.  Has anyone done this?