Outsourcing became a key business strategy in the 1990s and the industry continues to grow.
Researchers took into account a range of factors to measure this, including brand reputation, online engagement and revenue.
With eight entries, firms whose headquarters are located in the US dominate the top 20, with an estimated total revenue of US$36 billion.
Here, Business Chief takes a look at the top 10.
Founded in Quebec City, CGI initially stood for 'Conseillers en gestion et informatique', which translates to 'consultants in management and information'.
The company has grown exponentially over the years to become an international success story, specialising in outsourcing across the world.
CGI offers a high level of professional services, including in customer support, business administrative processes and the recruitment of highly-skilled employees with technical knowledge.
09: HCL Technologies
Based in Noida, India, HCL Technologies' operations span more than 40 countries across the globe in multiple continents.
Primarily, the firm's goal is to help transform and modernise businesses through its expertise in the fields of digital, engineering and cloud technologies.
In practice, services are offered to a huge variety of sectors, arming clients with workplace solutions which are said to be sustainable and cost-effective.
Headquartered in Bengaluru, India, it provides integrated outsourcing and transformation services.
Infosys has continuously been ranked as one of the world's leading BPM companies over the past five years. Outsourcing services are split into areas including customer service, analytics, finance and accounting, human resources and procurement strategy.
Emerson was established way back in 1890 as Emerson Electric Manufacturing Co. to manufacture electric motors.
Today, the firm is a global leader in the mechanical and industrial engineering sectors.
When it comes to outsourcing, Emerson works with clients in two ways. On the one hand, it offers assistance in the creation of automation services for a variety of fields and, on the other, it provides commercial and residential solutions for various heating and cooling appliances.
Founded back in 1967, Capgemini has made its presence felt in more than 40 different countries over the years.
The company is known today for providing qualitative services in technology, consulting and outsourcing.
One of the Capgemini's primary goals is to make outsourcing more accessible for mid-sized companies, offering an "affordable alternative to the rigid and expensive agreements typically offered by large outsourcing mega-vendors."
Their costs are billed as being 20% to 40% lower than competitors.
The highest US-based entry on the list, Cognizant bills itself as a global IT firm which transforms customers' business operations.
It is placed 194 on the Fortune 500 and is regularly recognised as being among the world’s most respected businesses.
Cognizant's outsourcing is focused on global industries such as communications, insurance, manufacturing, retail, healthcare and financial services. The company provides services including cloud enablement, quality assurance and software product engineering.
Coming in at number four on the OA500 is German technology company Siemens.
Interestingly, back in 2019, Siemens brought in fellow OA500 heavyweight Accenture to run and maintain a number of its data analytics and business intelligence services.
The programme saw Siemens outsource its data analytics and business intelligence function and team to Accenture, in a five-year deal that was seen as a significant expansion of the firms' ongoing partnership.
Originally based in the Indian city of Bengaluru, Wipro's services now reach the Americas, Europe, the Pacific, the Middle East and Africa.
Business process outsourcing (BPO) services offered include consulting, infrastructure services and data, analytics and AI.
This variety and competitive pricing have helped the firm to stand out above its competitors.
What's more, the company's commitment to sustainability has made it a popular choice given today’s economic climate.
Although headquartered in France, Teleperformance launched its outsourcing function in the Philippines in 1996.
The firm offers digitally-integrated business services to support customer experience management, business process services and back-office services.
Digital solutions are provided to several industries, including Retail e-commerce, Energy and Utilities, Healthcare and Financial Services.
In the Philippines, Teleperformance has 45,000 employees operating across more than 30,000 workstations, which are located at 22 sites across the country.
Accenture had its origins in the business and technology consulting unit of accounting firm, Arthur Anderson, before splitting off as a separate company in 1989 and adopting its current name in 2001.
Today, the company is a world leader in technology, consulting and outsourcing with 400,000 employees worldwide – 45,000 of which are based in the Philippines.
Accenture serves millions of clients across the world, but its traditional call centres have largely given way to to contact centres delivering high-value services.