Oklahoma! established the "book musical" format/formula that has been used as the basis for musical construction (and deconstruction) since its inception.
This format included:
Script as the spine of the show
Romantic main plot paralleling and contrasting a comic subplot
Integration of……
1st Act – music/character development
2nd Act – tying up the story.
Recurring music – unification
Specific Social Commentary
Other innovations:
Naturalistic acting
Four-part choral arrangements
Americana music
Ordinary people using ordinary language
Songs intertwined with dialogue, underscoring, and recitative