The geographic feature of the Ozarks that is not typically found in the Ouachita Mountains is the presence of "alluvial plains."
The Ozarks have significant areas of alluvial plains due to the river systems that flow through them, leading to sediment deposition and the formation of fertile lowlands. In contrast, the Ouachita Mountains are characterized more by their rugged, mountainous terrain and less by the flat, sediment-rich plains found in the Ozarks.
So, to summarize, "alluvial plains" is the feature primarily associated with the Ozarks that is typically not a feature of the Ouachita Mountains.