I took a quick look through https://books.google.com/ngrams/graph?content=%22open+source... and I couldn't see the term being used in a software context before 1998.
https://books.google.com/ngrams/graph?content=open+source&ca...
I took a quick look through https://books.google.com/ngrams/graph?content=%22open+source... and I couldn't see the term being used in a software context before 1998.