It depends on which calender you defaulted to on your iPhone.
Go to Settings>Mail, Contacts, Calenders>scroll down to Calenders section>Click on default Calender>place a check mark on whichever you would like to use.
Then in Outlook put a check mark in that same Calender. Uncheck the others.
With mine I cleaned it up. I have a check mark only on the iCould group and I use "Calender-iCloud". There was another Calender in that group I think was called "Home". I right clicked and deleted it. Everything defaults to the one calender now.