Here, a low-temperature solution-processed nickel oxide (NiOx) thin film was first employed as a hole transport layer in both inverted (p-i-n) planar and regular (n-i-p) mesoscopic organic-inorganic hybrid perovskite solar cells (PVSCs). In p-i-n PVSCs, the wetting properties, perovskite morphology, absorption and hole extraction process can be significantly enhanced with a suitable surface treatment, resulting in a significantly increased fill factor (from 0.684 to 0.742) and short circuit current density (from 16.73 to 20.66 mA cm−2). On the basis of the treated NiOx thin film, a promising power conversion efficiency of 15.9% with negligible hysteresis was obtained for inverted planar PVSCs, and 11.8% was obtained for the flexible devices. More importantly, the presynthesized NiOx can be directly deposited on the perovskite film as a top hole transport layer without decomposing the perovskite in n-i-p PVSCs. The resulting n-i-p device shows a five-fold improvement in power conversion efficiency when compared with a hole transport material free device, which indicates that this solution-processed NiOx is promising for all-inorganic charge selection layer based, stable and low cost PVSCs.