The study investigated the commercial activity in Oman during the sixth century of Hegira/ twelfth century AD through the book "Al Dheyaa" by Al Otabi. The study focused on monitoring and tracking the key aspects and various commercial forms in Oman, mainly those related to commercial markets and transactions in view of the fatwas and juristic issues contained in the Book "Al Dheyaa". The study aimed at identification of the key commercial centers and means of transport during the period, and exploration of the types of sales and key commercial transactions that prevailed at the era, such as loans, mortgage, guarantee and remittance. The researcher adopted the historical, descriptive and analytical methodology by induction of parts of the book "Al Dheyaa" related to commercial activity, and analysis of the texts of juristic issues and their application. Importance of the study stems from questioning the juristic sources to bridge the gaps that weren't addressed by the conventional historical sources. Choice of the book "Al Dheyaa" by Al Otabi is based on the fact that it is one of the key juristic works in the period of the study. It represents prosperity of the elements of authoring the juristic encyclopedia. Through its fatwas, it was possible to identify the aspects of economic life and commercial activity the subject of the study. The study consists of an introduction, four chapters and conclusion. Chapter one handles the life and era of Al Otabi, and discusses the importance of the Book "Al Dheyaa" and content of its parts as well as its historical and cultural importance. Chapter two investigated markets in terms of their management, regulation, types and factors that affected market movement and identification of the scales and money that was used at the time. Chapter three was dedicated to examination of trade centers and means of land and sea transport. Chapter four addressed the types and key commercial transactions, such as guarantee, companies, remittance, tithe system and taxes. The study drew set of conclusions, mainly: that the activity of commercial movement in Oman contributed to support the commercial relations with the neighboring countries and their regional and international periphery. The study indicated multiplicity of the methods of sale and purchase in Oman during the period of study, including sale in cash, bartering and credit. The study confirmed that markets, in addition to being place for sale and purchase, represented space and field of social and cultural interaction.