The process by which dense, star-forming clouds form from the diffuse ISM is an important part of galaxy evolution, and a major unsolved problem in modern astrophysics. To construct a comprehensive observational picture, multiple tracers are needed that can follow the ISM through all stages of its evolution. However, we lack large-scale spectral line tracers of the diffuse molecular regime, in which hydrogen is in the form of H2 but CO abundances are low. Such "transition-state" gas is the missing link that probes the earliest stages of the molecular cloud formation process, but is completely missed by traditional molecular line surveys. The ground state OH lines at 1612, 1665, 1667 and 1720 MHz may provide an answer to this problem. With low critical densities and enhanced abundances in diffuse molecular gas, OH is an effective - but underexploited - tracer of the CO-dark molecular ISM. SPLASH (the Southern Parkes Large-Area Survey in Hydroxyl) is a large, unbiassed and fully-sampled survey of OH emission, absorption and masers that will map the Galactic Plane and Galactic Centre at unprecedented scales and sensitivities. This dataset will answer critical questions on the global distribution of diffuse OH, the degree to which it traces hidden molecular gas, and its role as a probe of molecular cloud formation. As a blind and sensitive survey of all four ground-state transitions, SPLASH is also detecting a large number new OH masers, facilitating a broad range of astrophysical studies, from star formation, to evolved stars, to SNRs. I will report on the science aims and strategy of SPLASH, and present initial results from its first two semesters, which have revealed a rich and complex distribution of OH emission and absorption in a subsection of the Southern Milky Way.