Ofcom Online Safety Act guidance

Ofcom's Online Safety Act guidance is the body of documents UK regulated services have to work from: the illegal harms and children's risk assessment guidance, the codes of practice for user to user and search services, the register of risks, and the record keeping requirements. Together they define what a compliant service does and what evidence it keeps.

It's dense but it's the source. Any tool claiming OSA compliance is mapping to these documents.