Studying DNA hybridization equilibrium at atomistic length scales, either via molecular dynamics (MD) or through commonly used advanced sampling approaches, is notoriously difficult. In this work, we describe an order-parameter-based advanced sampling technique to calculate the free energy of hybridization, and estimate the melting temperature of DNA oligomers at atomistic resolution. The free energy landscapes are reported as a function of a native-topology-based order parameter for the Drew-Dickerson dodecamer and for a range of DNA decamer sequences of different GC content. Our estimated melting temperatures match the experimental numbers within +/- 15 degrees C. As a test of the numerical reliability of the procedures employed, it was verified that the predicted free energy surfaces and melting temperatures of the D- and L-enantiomers of the Drew-Dickerson dodecamer were indistinguishable within numerical accuracy.