Up to 78% of oil and gas megaprojects fail to deliver on initial commitments, yet maintenance readiness is routinely deferred until after construction decisions are already irreversible. This study investigates the previously unexamined link between maintenance readiness and interface management across the capital-project life cycle through a systematic literature review combined with a 30-year longitudinal analysis of 941 anonymised interface lessons-learned records spanning 21 countries. Root causes were classified using ISO 14224 of 2016, and predictive models were validated through stratified coding (Cohen's κ = 0.81) and five-fold cross-validation. The analysis revealed that five root causes drove 81.2% of all failures, including slow interface identification (18.5%), uncertain technical definitions (17.4%), weak governance (17.4%), unclear scope and ownership (16.3%), and cross-discipline communication breakdowns (11.6%), with key effects being performance impact (41.8%) and schedule delay (25.7%), while 33.0% of records were classified as high or critical risk and approximately 40.3% originated in Front-End Engineering Design. The Interface Criticality Index (ICI) correlated strongly with high-risk classification (r = 0.83), and the Random Forest classifier achieved ROC-AUC = 0.848 and PR-AUC = 0.850 respectively. The findings conclude that maintenance readiness should be established as a distinct, front-end-loaded discipline rather than a post-handover activity, as embedding readiness controls upstream converts lessons-learned records from retrospective artefacts into proactive design constraints, enabling early intervention before costly lock-in.