Wikipedia: O Canada/Talk

There are some additional verses but like those of other anthems they are poorly known and rarely sung.

Actually, this doesn't seem to be the case. The anthem has only one verse in both English and French. However, the poem that the English version was based on has several more verses; people sometimes use those as addition verses to the song, but they aren't really. --STG

In the 1980s it was common to mix the lyrics across languages, switching from English to French after the fourth line, then back to English after two lines in French.

I'm not so sure about that. I remember it going like this:

 O Canada! Our home and native land!
 True patriot love in all thy sons command.
 Car ton bras sait porter l'épée,
 Il sait porter la croix!                  
 Ton histoire est une épopée
 Des plus brillants exploits.              
 God keep our land glorious and free!
 O Canada, we stand on guard for thee.
 O Canada, we stand on guard for thee.

Two lines English, four lines French, last three English. --STG

: Correct. I was trying to remember how it went by singing it out loud at home -- good thing I'm single -- but now that I see it written down it's coming back. -- Paul Drye