A deflectable unilateral or bidirectional catheter for mapping and/or ablation, has a catheter body and a catheter tip section carrying a reference sensor and a scaffolding structure that is deployed for stabilizing and anchoring the catheter in or near the coronary sinus of the heart. In one embodiment, a control handle 16 at the proximal end of the catheter body 12 moves a sheath proximally to deploy the scaffolding structure. In a sheathless embodiment, the control handle moves a puller wire distally to deploy the scaffolding structure, which can be a generally spheroid shape, ovoid shape, basket shape, peanut shape and synaptic shape. The arrangement of the scaffolding structure, the reference sensor and a plurality of electrodes on the tip section can be varied as appropriate or needed.