Singing: live or dubbed?

Yes, the singing was pre-recorded and the actors had to lipsynch the songs. Dialogue was recorded whilst they were filming and this had its own problems (see http://www.kodak.com/US/plugins/acrobat/en/motion/newsletters/inCamera /oct99/oklahoma.pdf).

Filming was over 18 days between the end of the run at the National Theatre and the transfer to the West End. The live audience footage was recorded at an extra performance at the Olivier Theatre. You can read more about it in Maureen Lipman's book, Lipreading and also at http://www.pbs.org/wnet/gperf/episodes/rodgers-hammersteins-oklahoma/i ntroduction/113/

In contrast, Les Miserables was sung-through, where there were hardly any spoken dialog in a span of two-and-a-half hours of storytelling. Recitatives ( sung dialogs) were sung together with solos, duets, and ensemble songs. It would have been impractical to record that entire musical score and then lipsynch during filming.

Unlike in Paint Your Wagon, which was more of a traditional movie musical - the storytelling was a combination of general spoken dialog and the occasional songs.