St Mary's School in 1874 was listed as situated on the north west
corner of Maitland Street and Ottaway Avenue. Where is Ottaway Avenue
today?

Ottaway Avenue is now known as South Street.

"Nov 3, 1875 Lieutenant Governor of Ontario, D.A. MacDonald officially
opens the 56-bed London General Hospital on Ottaway Ave., now called
South Street."

London Health Sciences Centre
http://www.lhsc.on.ca/repcom01/4.htm

"His appointment coincided with occupancy of a new Medical School
Building on Ottaway Avenue (later known as South Street) across the
road from Victoria Hospital."

The University of Western Ontario: Department of Biochemistry 
http://www.biochem.uwo.ca/history.html

"One of the old street car routes travelled on Ottaway, now South
Street, a tribute to his beautiful wife's family name."

The London Free Press
http://www.lfpress.ca/perl-bin/niveau2.cgi?s=gallery&p=21870.html&a=1

"The four-story building located on Ottaway Ave, now renamed South
Street, adjacent to Victoria Hospital, was to provide laboratory
analysis services for area doctors and teach public health education."

The University of Western Ontario
http://communications.uwo.ca/anniversary/historic_story.html?listing_id=6650

You can see the location of South Street on various maps at MapQuest:

MapQuest
http://www.mapquest.com/maps/map.adp?searchtype=address&formtype=search&countryid=41&addtohistory=&country=CA&address=South+Street&city=London&state=ON&zipcode=&submit=Get+Map

Google search strategy:

Google Web Search: "ottaway avenue OR ave" + "london"
://www.google.com/search?hl=en&ie=UTF-8&q=%22ottaway+avenue+OR+ave%22+london

I hope this is precisely the information you need. If it is not, or if
a link doesn't work for you, please request clarification; I'll be
glad to offer further assistance before you rate my answer.

Best regards,
pinkfreud

Thank You. I was unable to find Ottaway Avenue through City of London.

I am sorry you were not fully satisfied by my answer. The reason you
are not able to find Ottaway Avenue, as mentioned above, is because
its name has been changed to South Street. The position of South
Street in the city is plainly indicated by the various MapQuest
graphics. What more do you require?