In recent years, major commercial publishers have strengthened their presence in both the subscription journal market and the open access journal market. Examining 447 journals from Elsevier and 550 from Springer Nature, this study investigates three strategies for enlarging the number of gold open access journals: the launch of new journals, mergers with other publishers, and partnerships with research institutes. The results reveal that these publishers adopted different strategies for expanding their journal portfolios. While Springer Nature relied significantly on merging with established publishers, Elsevier recently launched many new journals independently. Approximately 60 per cent of Springer Nature journals and 45 per cent of Elsevier journals are published on behalf of research institutes. Therefore, collaboration with research institutes has contributed to the increasing number of journal titles. As major publishers expand their open access businesses, it is necessary to monitor their activities from a policy perspective of pro-competition.