An impressive library of functional oxide nanomaterials is now available, encompassing multicompartment nanoparticles and heterostructures in general that enable further tuning of the properties. Nonetheless, the vast majority of these nano-objects is built on relatively common oxides, easily synthesized at the nanoscale by chemical methods known since decades. This range of nanoscaled oxides is narrow compared to the library of bulk oxides. Extending the portfolio of oxide compositions at the nanoscale is mandatory to strengthen research efforts on the role of the nanoscale on the properties, and on the emergence of new properties. This chapter provides an overview of the synthesis techniques developed since few years to address complex oxide nanomaterials, encompassing multicationic oxides, oxides of metals with uncommon oxidation states, and new oxide crystal structures accessible only at the nanoscale. For each case, we illustrate the relevance of targeting such complex oxides by discussing their specific properties.