what's the difference?

That would depend upon the context--although the two words might be used interchangeably in many cases.

"He had the opportunity to make a lot of money."

"He had a chance to make a lot of money."

Apart from that, opportunity means a situation which one might be prepared to take advantage of: "He saw an opportunity to make a lot of money, and he took it." Chance can mean simply an accidental occurance. "He chanced to see a wallet lying on the sidewalk, as he was walking home."

Such questions are difficult at best, if you don't provide context.

Acknowledgement
Thank you very much,Setanta !

You're welcome--come back if you have more questions.