In this article the authors explored different approaches to assessing synergies in mergers and acquisitions of companies, such as: evaluation based on discounted cash flow; evaluation based on a combination of financial and operational multipliers, and evaluation based on changes in cash flow and brand strength. It is noted that when choosing approaches to evaluation synergic effect resulting from mergers and acquisitions, it is necessary to take into account specificity of oil and gas companies. The article considers internal and external factors that determine the specificity of the oil and gas company as exemplified by «Bashneft, JSC».