There are 10 words that start with 'opto'.
| optometer | optometry | 
| optometers | optometric | 
| optokinetic | optometries | 
| optometrist | optometrists | 
| optoelectronic | optoelectronics | 
Note: These 10 words that start with 'opto' are found from the USA & Canada version of the tournament Scrabble word list.