There are some osm2psql developers and users who have written up howtos or blog posts, gave talks, filmed tutorials etc. Not all of that is up to date with current version of the software and recommended usage, but there is a lot to learn there.
See also the osm2pgsql page on the OSM wiki.
- Jochen Topf writes occasionally about osm2pgsql in his blog.
- Ryan Lambert of RustProof Labs has a blog with lots of osm2pgsql content.
- From SOTM 2019: “OSM data processing with PostgreSQL/PostGIS” by Jochen Topf. Slides, Download Video, Video on Youtube.
- From SOTM-US 2014: “osm2pgsql:mapnik — Optimizing the Rendering Toolchain” by Paul Norman. Video on Youtube.