This work presents the first report of thermal atomic layer deposition (ALD) of NiO_x using two nickel precursors – Alanis™ and [Ni(ⁱpki)₂] – in combination with two different oxygen sources (H₂O and O₃), over a temperature range of 75–250 °C. The Alanis/O₃ composition exhibited one of the highest growth rates per cycle (1.1–1.4 Å) and a broad ALD window between 100 and 200 °C. In contrast, the Alanis/H₂O and [Ni(ⁱpki)₂]/O₃ combinations yielded lower growth rates of 0.74 Å at 150 °C and 0.40 Å at 250 °C, respectively. Comprehensive structural, morphological, optical, and chemical characterisation revealed that the choice of precursor combination and the reaction temperature significantly impact the film composition, thereby strongly influencing its suitability for various applications. Notably, those parameters closely determined the photoelectrochemical performance and the stability of the Si/NiO_x-based photoanode towards the oxygen evolution reaction (OER). Photoelectrodes fabricated with Alanis/O₃ at 200 °C demonstrate stability exceeding 24 hours and exhibit a remarkable OER onset potential of 1.15 V vs. RHE for a photocurrent density of 1 mA cm⁻².