The mode of education has changed over the past few years from the conventional method of in-person classes to the usage of online platforms to facilitate teaching and learning. These platforms popularly, known as online learning systems, have gradually become an integral part of education. These online platforms have been designed using various Artificial intelligence frameworks and techniques to enhance their functionality and personalize them for their users. Machine learning is one of the major fields of AI that has been used in most of these online platforms. Popular machine learning techniques such as deep learning, natural language processing, reinforcement learning, and others are being actively used and studied to further improve them for use. In this study, the focus will be on content analysis of different studies aimed at disclosing machine learning techniques that have been applied in the online learning sector and exploring the potential research trends and challenges of integrating machine learning techniques in online learning. The study will focus on published papers from the year 2015 to 2021, classifying them based on the research question.