We are developing a new hybrid atom-ion trap to study the interaction of ultracold rubidium atoms with mass-selected OH⁻ molecules. The ions are trapped inside an octupole rf-trap made of thin wires instead of the commonly used rods. This ensures good optical access to the center of the trap where the ions can be overlapped with laser-cooled rubidium atoms stored in a dark spontaneous force optical trap (dark SPOT). This setup provides high collision rates since the density in a dark SPOT is about one order of magnitude higher than in a standard magneto-optical trap. Further, inelastic collisions with excited atoms are suppressed since almost all atoms are in the ground state. Numerical simulations of our setup using SIMION predict that cooling of the ions is feasible.