This study examines to what extent how oil movements differently affect equity returns in general and sectoral levels of the GCC countries stock markets. Modeling the equity returns volatility requires using GARCH-type models. These models help to explore the pronounced differences of the conditional variance structures across sectors and markets. Chapter 1 compares the effects of changes in oil price return and its volatility on equity returns and volatility across sectors. The findings of this chapter show that despite the GCC states dependency on oil revenues, equity market performance at the sectoral level do not exactly associate with oil movements. Our results, in particular, show that the GCC stock markets do not always move hand-in-hand with oil market movements. In chapter 2, we explore the relationship within a specific sector, i.e. Banks sector in Saudi Arabia Stock market. We examine if oil price changes affect Islamic banks differently than conventional ones. The findings show a decrease in degree of co-movement between these two types of banking system and oil market, meaning that they are less integrated. Although the Islamic banks kept a higher degree of co-movement with oil, limitations of Shari'ah restrictions on Islamic banks have little impact on the relationship between oil and those banks. Chapter 3 examines whether the level of corruption influences how oil changes affect the GCC stock markets. The findings of chapter 3 show that dissimilar levels of corruption between GCC countries have inconsiderable differences on the oil return effects on the GCC stock markets. Oil returns affect both low and high level of corruption groups. The oil return innovation affects the equity volatility for Saudi Arabia and Kuwait more than other four GCC countries.