The paper is well written and organized. The data is collected following a rigorous manner. However, certain changes need to be made before it is ready to be published:
1. The paper gives a strong emphasis on the term “oligopoly”, which is a term appeared most frequently in economic literature about market competition. However, the current paper is following a descriptive style, reporting the revenue of top 5 big publishers, which is not sufficient to address market power, neither competition. Hence, the term “oligopoly” could mislead readers and put on expectations, which this paper does not intend to fulfill.
2. In subsection 3.1.3, the 3rd paragraph: “For the remaining journal-year combinations missing a fee, we applied APCs from a year for which we did have data. In an attempt to under- rather than overestimate fees paid, we prioritized older data from Matthias (2020). We applied APCs using older data from Matthias (2020) in 398 cases (Table 2: “Matthias older”) and older data from a website snapshot via Internet Wayback Machine for 4 journal-year combinations ("IWM older”). For journals without any older APCs, we applied more recent APCs from Matthias (“Matthias newer”, n=371), Morrison (“Morrison newer”, n=40) and Wayback Machine (“IWM newer”, n=4). For 88 (0.5%) journal-year combinations we had to use current APCs from publisher websites due to a lack of information from other data sources.”
Instead of using the APC information of the wrong years, the authors can consider extrapolate and/or interpolate the APCs based on previous and/or future APCs of the same journals by e.g. regressing the APCs on years of a curtain to get the year trend of the pricing.
3. In subsection 3.1.3, the 4th paragraph: “Since our manual check suggested that both data sources were correct some of the time, we chose not to prioritize one source over the other and use the lower APC amount in case of conflicts.”
Instead of choosing one value over another in this case, the authors can consider using time-weighted means, i.e., the APCs are constructed proportionately according to the time lengths of different conflicting prices. If the authors do not have the information of the time lengths, then why did the authors not simply take the means?
4. There are several places, where the term “cost” is used but meant differently. It can be confusing for readers.
In subsection 3.1.3, the 5th paragraph: “The cost of OA was determined by multiplying the lower APC list price for a particular journal-year combination with the number of gold or hybrid OA articles published that year as determined by Unpaywall.” And in subsection 4.3, the 1st paragraph: “Figure 6 displays the total fractionalized costs per country for the 25 countries with the largest amounts of APCs.” The “cost” in these two places means the cost for authors, which can be alternatively mentioned as e.g. the expenditure of the academic community on OA. Nonetheless, in section 5, the 4th paragraph: “The Fair Open Access Alliance (FOAA), an organization that evaluates sustainable OA publishing, analyzed the cost to publish OA and demonstrated that much lower APCs of no more than $50 per page are feasible to sustain an OA journal.” The “cost” here means the production cost for the publishers. These first two scenarios are referring to the price and the last scenario is the cost.
5. Figure 3 shows the shares of gold and hybrid APCs paid by the scholars to the top 5 publishers, which is also a piece of information contained in Figure 1, but shown in different way. Figure 3 is comparatively not informative.
6. It might be helpful for readers to grasp the information more quickly in Table 3, if authors could sort the publishers according to the number/share of gold articles without APCs.
7. In Table 4, the APC of Elsevier in 2016 has no currency sign. Addtionally, it might be more straightforward to show the trend of APCs by displaying line charts, rather than showing so many 4-digit values in a table.
8. There two places, where the word “above” is used: in subsection 4.2, 2nd paragraph: “…Elsevier, all except one of which were hybrid, reflecting the publisher’s OA portfolio described above.” And in subsection 4.4, 2nd paragraph: “Figure 8 shows the share of publications per OA status per discipline, reflecting the high share of hybrid articles in the Humanities and Social Sciences described above.”
The authors might consider to be more specific, e.g. to point out in which section and/or paragraph.
9. In section 5 discussion, there are several places, where the discussion is extended to the content, which cannot be supported by the descriptive analysis of the paper, neither can it be seen well-connected to the rest of the paper. The authors might consider to trim the discussion scope down, so that the discussion can connect to the analysis better.
a. Section 5, paragraph 1: “Although OA was meant to remove inequities and break the oligopoly’s control of the traditional scholarly market…”. In the previous parts, OA is explained to offer a free channel for knowledge access, which will make knowledge affordable for scholars. However, this does not necessarily mean equal for publishers or breaking their dominant situation. If this claim is stated in the open access initiative, the authors might consider to mention and explain it in the previous parts of the paper.
b. The following sentence: “These publishers are now exerting their power in the OA system” together with the 3rd paragraph: “That market power translates to APCs, where publishers apply tactics like price discrimination, charging higher APCs and subscriptions for high-impact journals….” The current paper studies the revenue of publishers generated through OA articles, which cannot directly be seen as market power.
c. Section 5, the 3rd paragraph: “The significant amounts of revenues the oligopoly draws from the OA market suggests that these publishers were able to use their dominance in the traditional market to quickly establish themselves as leaders in the emerging OA market in the face of competition from new players such as PLOS, MDPI, Frontiers and others.” The current study does not include neither any analysis of the traditional market, the market power in traditional market, nor the market dynamic analysis connecting the traditional market to the OA market.
d. In the same paragraph: “Mergers and acquisitions as well as general growth of publication output led to further consolidation of the market, allowing these publishers to control APC prices.” It is a claim the current analysis does not support, nor is it relevant to the rest of the paper.
10. In section 5 paragraph 5, the authors equate reputation, prestige and citation. Some literature use citation or impact factor as the proxy for journal quality. Some literature argues this journal quality is a proxy for reputation. But authors might need to be more cautious to equate these three concepts right away, especially with prestige. There are plenty of journals with historically long-established reputation (which might be prestige in this case?) having relatively lower citation scores or impact factors, compared to many younger and less community-recognized journals.
11. The same paragraph: “A look at the publishing landscape in the Global South demonstrates that sustainable and affordable OA publishing exists outside the author-pays model (Khanna et al., 2022). More than two thirds of OA journals indexed in the DOAJ do not charge…”
The authors might need to be more cautious, when pushing policy makers to set 0 APCs as your goal, which is pushing the current over-charging extreme situation to another extreme. The current OA journals charging 0 APCs could be also offering lower quality than the non-zero ones, since publishing, with as low cost as it could be, cannot result in absolute zero cost. 0 APCs is affordable and sustainable OA publishing for authors but not sustainable for publishers. Wouldn’t be a solution, which is sustainable for both authors and publishers, a better solution?
12. When the scope of discussion is trimmed, then the conclusion should be considered to adapt accordingly, in order to connect to the rest of the paper better.
13. Some final small remarks: The authors should proofread the paper at least one more time. When the authors cite and describe others’ work, some places use past tense and some places present tense, e.g. “Pollock and Michael (2021) estimate that…”, “Simard et al. (2021) estimated that…”… There are places, where a comma is needed, but missing and one place has a left double quotation mark, which is not needed. At one place, the first letter is not capitalized.

I think the revised manuscript is acceptable for publishing.