As a new family of quasicrystals, oxide quasicrystals (OQCs) were recently discovered in Ba-Ti-O films on Pt(111). Here we report on the growth and composition of an ultrathin Ba-Ti-O OQC film and its crystalline approximant, studied by scanning tunneling microscopy (STM), low-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. STM images reveal a wide-scale ultrathin OQC film without BaO islands, and magnified STM images display typical OQC clusters. The ultrathin OQC film has been prepared by annealing an oxide crystalline approximant with decreasing Ti concentration. We also found that the ultrathin Ba-Ti-O film forms a $(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure on annealing the ultrathin OQC film. For quantitative analysis, the XPS peak intensity was measured as a function of elemental atomic density. The elemental atomic densities for the ultrathin OQC film were determined to be $8\ifmmode\times\else\texttimes\fi{}{10}^{14}$, $4\ifmmode\times\else\texttimes\fi{}{10}^{14}$, and $3\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}/{\mathrm{cm}}^{2}$ for Ba, Ti, and O, respectively.