Organisational Health Literacy (OHL)-interventions are needed to overcome health inequality. OHL-interventions have successfully identified communication barriers at the organisational level, but evidence is limited on the extent to which this leads to sustainable organisational change. This study aims to assess the implementation fidelity, moderators (barriers and facilitators), and long-term impact of OHL-interventions in hospitals in Ireland and The Netherlands. We used a longitudinal mixed-methods approach to assess two similar OHL-interventions in one Irish and three Dutch hospitals. The OHL-interventions concerned the improvement of navigation and implementation of health literacy-friendly communication throughout organisations. Participants were 24 hospital employees and 40 older adults who use hospital services. At six, eight, and eighteen months, we assessed the level of implementation, barriers and facilitators, and impact through questionnaires and in-depth semi-structured interviews. After older adults and professionals had identified a number of communication problems, we found that professionals had successfully implemented OHL-interventions to promote navigation and comprehensible communication. Limited resources and variation in organisational structures and procedures were perceived as barriers to implementation. The participation of service users, leadership support, and a stepwise implementation of interventions were perceived to facilitate implementation. In the long term, the OHL-interventions led to system-wide improvements, as shown by better embedding of health literacy policies, enhanced patient engagement, provision of plain language training and comprehensible information. Findings were similar for the two countries. Embedded OHL-interventions resulted in sustainable and system-wide health literacy changes in all four hospitals. Following implementation, OHL-interventions have the potential to promote health equity and empowerment among health service users.