I've read about OCEAN (or The big five or FFM) on Wikipedia and I'm intrigued. I would like to have a categorisation of attributes describing a personality according to the model. Since we have a lot of attributes and this is a potentially subjective for each attribute if you were to add any measure of intensity, I wanted to ask if you know any source, preferably a scientific one, where this has been done already. (A non-scientific source could be a categorisation done in a blog about character design in novels, for example.)
It's unclear what is your definition of attributes. But the Big Five emerged from the lexical hypothesis whereby adjectives in the dictionary that describe people were systematically examined and categorised using approaches such as factor analysis. Some of the papers in this area would contain lists of terms.
Equally, for briefer lists, you could look at the domain-facet frameworks of various Big 5 personality models, such as the NEO. In this context, the facets help to define the Big 5 Domains.
Here are a few such domain-facet/aspect frameworks, summarised in Anglim & O'Connor (2018, PDF). That article also provides a brief review of the lexical approach with some further references.
Anglim, J., & O'Connor, P. (2019). Measurement and Research Using the Big Five, HEXACO, and Narrow Traits: A Primer for Researchers and Practitioners. Australian Journal of Psychology, 71, 16-25. https://psyarxiv.com/a78g2/download