Oklahoma Native Plant Society - "The purpose of the Oklahoma Native Plant Society is to encourage the study, protection, propagation, appreciation and use of Oklahoma's native plants." In order to encourage the use of native plants in Oklahoma landscapes and small gardens, they provide a number of resources, including field trips, an Oklahoma native plant record, and membership information.
Oklahoma Wildflowers - On this site, you'll find a decently large collection of landscaping photos featuring wildflowers found in the state of Oklahoma. Each picture is identified by its common name and, to help you identify your own wildflower finds, there are links to other flower identification resources.
Catalog of the Woody Plants of Oklahoma: Descriptions and Range Maps - This page contains information about woody plants found in Oklahoma. "For the purpose of this document, a woody plant is defined as a plant that retains some living woody material at or above ground level through the non-growing season." Included within this site is a list of such plants, listed by common and scientific names. The individual pages for the plants provide ample data about the plant in question, including a description of the plant and where you will typically find it growing in the wild.