In this study, high-performance multilayer WSe₂ field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe₂ FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe₂ thickness. The carrier type evolves with increasing WSe₂ channel thickness, being p-type, ambipolar, and n-type at thicknesses < 3, ~4, and > 5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe₂ as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe₂ FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.