The preferential growth phenomenon of the polycrystalline YBa2Cu3Ox seed film deposited on (100) MgO substrate was investigated using liquid phase epitaxy (LPE). The x-ray diffraction and the polarized Raman spectroscope were used to evaluate the in-plane and out-of-plane orientation of as-grown films. It is found that a so-called 45° oriented in-plane alignment preferentially formed under the pure oxygen atmosphere, which is dissimilar to the 0°-oriented grains presenting under the air environment. The mechanism study on the initial stage of REBa2Cu3Ox (REBCO, RE=Y, Nd) LPE growth was performed, in which the morphology evolution shows a breaking coalescence of 45° islands by entrapped fluxes in an intensive manner in the 45° LPE film. The peak around 230 cm-1 in Raman spectrums was detected only in the 45° LPE film, indicating relatively weak grain connectivity, which agrees well with broken coalescence structure. Moreover, a general comparison comprising interfacial terminal layer, oxygen content, geometrical coherence, the minimum energy cut, and experimental results from different REBCO systems was made for comprehensive understanding. It highlights the significance of oxygen content status in choosing the most stable interfacial layer and elucidates the origin of the preferential 45°-oriented growth under pure oxygen condition.