How are scientific papers uniquely identified? Some papers have the Digital Object Identifier (DOI) which uniquely identify them, some other have an oai identifier (e.g. oai:arXiv.org:1402.3722). Even though I inspected the metadata of different sources like core, microsoft academic, and semantic scholar I couldn't find a unique identifier which is used among all sources of research papers.

How are all scientific publications uniquely identified?
Research publications predate the digital age, so only a fraction has a digital identifier. The unique identifier traditionally used is the full citation, of which various formats exist to suit discipline-specific needs. It is very likely for a full citation to be unique.

(However, automatic data analysis may not be able to recognize the identity of diffently formated citations of the same publication. That's DOI's strong suit.)