A thrilling* place to visit!
http://www.pc.gc.ca/eng/pn-np/bc/yoho/ ... ohara.aspx
For bus plus camping, reservations open April 1 for 3-months-in-advance to the day.
For bus only, reservations open April 20 for entire season.
If you can't get either, the initial (dirt) road walk of 7 miles is not too onerous; the last bus, ca. 4 p.m., will take everybody back for ca. $10 Canadian.
Nearby sleeping spots are Whiskey Jack Hostel, and near it, the Takkakaw Falls walk-in campground (they provide a wheelbarrow to haul your stuff).
(*When I visited, campers were sitting around listlessly in waterlogged tents.)
As I recall, the Parks Canada info center in Field can check for a cancellation if you suddenly decide you want to camp at Lake O'Hara.