Identifying Open Access (OA) publications might seem a trivial task while practical efforts prove otherwise. In this project, we wanted to assign OA tags to publications in KB database. We queried KB in-house database up to 2017 (including Web of Science (WOS) and Scopus) for all articles and reviews. We then matched the corresponding DOIs to three sources of OA information: Unpaywall, Crossref and Bielefeld list of gold OA journals. This allowed us to define the OA status for publications. We found close to 14 million publications (articles and reviews between 2000 and 2016) from WOS (69.75% of all) and close to 18 million from Scopus (68.67% of all) with an equivalent DOI in Unpaywall. We matched KB publications database with Crossref data (from April 2018) and found 53 distinct licence URLs, which define in many cases the legally binding access status of publications. We found that more than half a million publications have more than one licence record in Crossref (in contrast to near 8 million with only one record and more than 6 million without a licence URL). We evaluated if these licences were open or closed access. We also matched respective journal ISSNs with DOAJ and ROAD databases and presented a categorization of publications to Gold, Hidden Gold, Hybrid and Delayed OA accounting for uncertainty due to missing licence information via a new sub-category Probable Hybrid OA. We validate our findings via manual checks and a crosscheck of OA information from the aforementioned varying sources. While the manual check on a sample of publications revealed a small but noticeable degree of apparently incorrect meta-information on publication’s OA status, the contrast of OA information from the diverse OA information sources highlights the partially unsteady base for an OA monitoring based on open data.