The clinical applications of optical coherence tomography (OCT) have expanded beyond ophthalmology to include imaging of various body cavities, functional imaging, diagnostic integration, and surgical robotics. Consequently, there is an increasing need to enhance the resolution of OCT to analyze biological tissues at the cellular level and beyond. However, enhancing OCT's lateral resolution typically results in a reduced depth of focus, which complicates the acquisition of a consistent point spread function throughout the imaging range. To address this issue, researchers have developed various strategies for extending the depth of focus, including using Bessel beams, phase masks, dispersion effects, synthetic apertures, computational imaging, multimode interference, and dynamic focusing. Each method employs distinct principles to improve the depth and quality of OCT imaging. This study thoroughly examines the principles, implementation methods, benefits, and limitations of these techniques. In addition, this study compares the performances of different depth-of-focus extension methods in the contexts of free-space and endoscopic OCT. Finally, this paper offers insights into future developments in depth-of-focus extension techniques for OCT, highlighting promising research directions and potential challenges.