Laws are typically published two ways: chronologically as they were passed, and topically by subject. The Oklahoma Sessions Laws are a chronological record of every law passed by the Oklahoma Legislature in a particular legislative session.
Typically, you will not start your statute research in these session laws. Instead use the Oklahoma Statutes or Oklahoma Statutes Annotated to find relevant statutes. Session laws are usually needed for historical research, to determine how a law has changed through the years.
By "all laws", this means that the books include laws which aren't "permanent" (for example a budget appropriation) or aren't of general applicability (for example a law which only affects one city or county).
Please note that prior to the 1960s the Oklahoma Legislature only met every other year, so the volumes will appear to skip years.
Also note that the Oklahoma Sessions are available online at http://www.oscn.net under "Legal Research". These are not official, and only go back to 1998.
Please see the Bluebook, or Appendix 1 and 2 of the ALWD for specific citation rules, but generally:
1975 Okla. Laws [page number].