New viral approaches have revolutionized neuroscience by precisely delivering genes in neurons; for example, to control or monitor activity in specific neuronal cell types. In contrast, the manipulation of oligodendrocytes requires the Cre-LoxP system and gene-by-gene engineering, breeding, and genotyping. Here we introduce OASIS (Oligodendrocyte AAV-CRISPR mediated Specific In vivo editing System), a versatile platform that combines SELECTIV, an AAV-receptor-based transduction strategy, with HiUGE, an NHEJ-mediated CRISPR/Cas9 knock-in approach. We show efficient and specific oligodendrocyte transduction across the brain and tagging of endogenous cytoskeletal, myelin, cell adhesion, scaffolding, and junctional proteins. OASIS enables sparse yet reliable labeling, allowing direct visualization of a protein's subcellular localization with single-cell resolution. We successfully fused the biotin-ligase TurboID with endogenous oligodendroglial Neurofascin-155, thereby achieving targeted biotinylation of the axoglial junction. OASIS is rapidly customizable for any gene-of-interest. Together, OASIS overcomes longstanding barriers in oligodendrocyte biology, providing a powerful system for precise, customizable genome editing and subcellular visualization in the adult brain.