Understanding the role of the oxygen content in silicon suboxides (SiOx) is important for the improvement of their performance as promising negative electrode materials for Li-ion batteries. Furthermore, sufficient research has not been conducted on tailoring the oxidation of silicon suboxides to employ them as anodes. To address these limitations, we demonstrated the lithiation of SiO2 and amorphous SiOx up to four Li atoms per SiOx and performed density functional theory calculations to examine the energetics, structural evolution, charge effects, mechanical properties, and volume energy density of lithiated a-SiOx. Our calculations show that two Li defects effectively break a Si-O bond in SiO2 and a single Li defect is easily stabilized by oxygen in a-SiOx. With lithiation, oxygen-tailored SiOx exhibits a difference in volume change and in voltage profiles. The tradeoff between the decreasing volume change ratio and increasing potential as the O content increases results in varied volume energy densities. The detailed reduction mechanism of Li in the a-SiOx system was analyzed by calculating the Bader charge. Our results emphasize the importance of tailoring the O content to obtain desired lithiation properties and give the physical insight of the development and rational design strategy of high-performance SiOx anodes dependent on the oxidation conditions.