There are 13 words that start with 'opin'.
| opine | oping | 
| opined | opines | 
| opining | opinion | 
| opinions | opinioned | 
| opinionated | opinionative | 
| opinionatedly | opinionatively | 
| opinionatedness | |
Note: These 13 words that start with 'opin' are found from the USA & Canada version of the tournament Scrabble word list.