A former teacher and basketball coach who's already facing dozens of sexual assault charges is facing 11 more after four more alleged victims have come forward, Ottawa police said.

Donald Greenham, 73, has previously been charged with sexual assault-related offences involving former students who were between the ages of 13 and 17 at the time of the alleged crimes. The allegations date back to the 1970s and early 1980s, police said.

Greenham was teaching and coaching for the Ottawa-Carleton District School Board at the time, police said.

On Wednesday police laid 11 fresh charges against Greenham.

The charges relate to four male victims who were between the ages of 12 and 16 at the time of the alleged offences.

The incidents occurred in Ottawa between 1972 and 1981, while Greenham was in "the course of his duties as a teacher and coach with the Ottawa-Carleton District School Board," police said.

Police say they're concerned there could be still more victims.

Greenham had also worked with youth groups in Brockville, Ont., and was a teacher in North Bay, Ont., an Ottawa police spokesperson said.