The necessity for long-term treatment of chronic diseases has enabled the development of long-acting injectable (LAI) formulations to improve drug pharmacokinetics and overall therapeutic effectiveness. A range of LAI formulations have been developed using various oils, lipids, and polymers alone or in combination. Classical oil-based long-acting depots are parenteral drug formulations designed to sustain the release of primarily lipophilic compounds. In this chapter, we discuss a few relevant aspects of oil-based solutions LAI properties, drug-to-formulation considerations, and composition elements contributing to release mechanisms. We have also outlined the formulation strategies of classical oil-based solutions, oil suspensions, and oleogels and their respective sterilization methods. Furthermore, we have also summarized the landscape of various oil-based formulations in clinical trials and marketed products.