Oklahoma is OK...and so is a modest but important collection of online content from the Oklahoma State Archives.
A selection of archived Collection Highlights has descriptions of mostly offline content, but one worthwhile online document:
Index to Oklahoma Confederate Pension Records includes full name, spouse's name, date of death, and microfilm identifcation information.
At the Oklahoma Digital Prairie you'll find diverse collections of documents, images and artifacts, including:
At the Oklahoma Historical Society you'll find yet more descriptions of offline materials, along with a few nice online gems, especially for family history research: