Two dimensional (2D) materials are extraordinarily sensitive to their surroundings, and several schemes have been demonstrated to dope 2D semiconductors using nearby ions, molecules or compounds without introducing chemical or structural defects that would degrade charge carrier mobility. Of these, oxides are attractive due to their stability and ability to function as dielectrics or passivation layers. Molybdenum oxide in particular has been demonstrated as a dopant for multiple 2D semiconductors. A practical oxide doping scheme requires not only deposition without damage to the 2D substrate, but also quantitative control over the doping density (e.g. via oxide Fermi level), which is currently lacking. Oxide work functions are sensitive to composition in general, and particularly so for molybdenum oxides. Thus, control of oxide stoichiometry could enable precise control of the doping level in 2D semiconductors, an approach which we demonstrate here. By using a low-oxidation-state molybdenum precursor (Mo(NMe₂)₄) for atomic layer deposition(ALD), we demonstrate two new low-temperature ALD processes to produce a series of molybdenum oxide compositions. The MoO_x optical bandgap and resistivity increase with increasing oxygen content, following the same trends as crystalline molybdenum oxides. We demonstrate controllable doping of MoS₂ by depositing a MoO_x overlayer and tuning the threshold voltage shift in thin film transistors (TFTs) from positive (p-type) to negative (n-type) by tuning the oxide composition. Low hysteresis in sweeps of the transistor gate voltage, the absence of strain, and reversibility upon oxide etching together suggest that this process maintains the van der Waals interface without significant chemical changes, and should thus be generally applicable as a doping strategy for 2D systems. Furthermore, the sequential deposition of spatially patterned ALD layers enables the fabrication of lateral p-n junction diodes.