Canonical duplex RNA assumes only the A-form conformation at the secondary structure level while, in contrast, a wide range of non-canonical, tertiary conformations of RNA occur. Here, we show how the 2′-hydroxyl controls RNA conformational properties. Quantum mechanical (QM) calculations reveal that the orientation of the 2′-hydroxyl significantly alters the intrinsic flexibility of the phosphodiester backbone, favoring the A-form in duplex RNA when it is in the base orientation and facilitating sampling of a wide range of non-canonical, tertiary structures when it is in the O3′ orientation. Influencing the orientation of the 2′-hydroxyl are interactions with the environment as evidenced by crystallographic survey data, indicating the 2′-hydroxyl to sample more of the O3′ orientation in non-canonical RNA structures. These results indicate that the 2′-hydroxyl acts as a “switch” both limiting the conformation of RNA to the A-form at the secondary structure level, while allowing RNA to sample a wide range of non-canonical tertiary conformations.