Abstract: The control of oxygen is paramount in achieving high-performance titanium (Ti) parts by powder metallurgy such as metal injection molding (MIM). In this study, we purposely selected the Ti and Ti–6Al–4V powders as the reference materials since these two are the most representative Ti materials in the industry. Herein, hydride–dehydride (HDH) Ti powders were pre-oxidized to examine the effect of oxygen variation on the characteristics of oxide layer on the particle surface and its resultant color feature. The results indicate that the thickness and Ti oxide level (Ti⁰ → Ti⁴⁺) of the oxide layer on the HDH Ti powders increased as the oxygen content increased, leading to the transition of color appearance from grey, brown to blue. This work aids in the powder feedstock selection at the initial stage in powder metallurgy. In addition, the development of oxygen content was comprehensively studied during the MIM process using the gas-atomized (GA) Ti–6Al–4V powders. Particularly, the oxygen variation in the form of oxide layer, the change of oxygen content in the powders, and the relevant parts were investigated during the processes of kneading, injection, debinding, and sintering. The oxygen variation was mainly concentrated in the sintering stage, and the content increased with the increase of sintering temperature. The variation of oxygen content during the MIM process demonstrates the crucial role of powder feedstock and sintering stage in controlling oxygen content. This work provides a piece of valuable information on oxygen detecting, control, and manipulation for the powder and processing in the industry of Ti and its alloys by powder metallurgy.