In this study, the structural, electronic, and vibrational properties of the thinnest crystal structure that can be obtained by thinning bulk Cd down to a monolayer are investigated by performing first-principles calculations. Total energy optimization and dynamic stability calculations reveal that the single layer crystal structure has a hexagonal unitcell with a two-atomic basis where alternating layers are formed by trigonal arrangements of Cd atoms. Softening occurs with decreasing zone center optical phonon frequencies as a result of structural relaxation when going from a bulk to a single layer (SL) structure. It is also shown that the thinnest structure obtained from bulk Cd crystals maintains its metallic features despite the dimensional crossover. In addition, it is predicted through calculations that the SL Cd crystal strongly interacts with oxygen and that the oxidized regions even undergo chemical transformation to form a CdO crystal. In the double-layer CdO crystal resulting from the oxidation of individual Cd layers, the layers are connected to each other with partially covalent bonds, and this structure is a semiconductor with a band gap of 2.10 eV. On the one hand, the robust metallic structure of the thinnest possible Cd crystal provides flexibility for its use in nanoscale applications, on the other hand, the fact that its electronic properties can be changed by oxidation is important for optoelectronic device applications.