The moisture sensitivity of wood-based reinforcement can restrict dimensional stability and durability in humid service environments. This study examined the effect of waste engine oil impregnation on the moisture-absorption behaviour of three tropical hardwoods—White Safari (Triplochiton scleroxylon), Opepe (Nauclea diderrichii) and Bush Mahogany (Khaya ivorensis)—considered for reinforcement in wood-based composite applications. Prepared specimens were separated into untreated controls and waste-engine-oil-treated groups. The treated specimens were immersed in filtered waste engine oil, drained and conditioned to stable mass before gravimetric moisture-absorption assessment. Moisture transport was interpreted using Fickian diffusion principles, while treatment effects were evaluated statistically at a 95% confidence level. Waste engine oil treatment reduced equilibrium moisture absorption in all three species. White Safari decreased from 11.80% to 6.20%, Opepe from 10.50% to 5.70%, and Bush Mahogany from 10.30% to 5.50%, corresponding to reductions of 47.46%, 45.71% and 46.60%, respectively. The treated specimens also showed slower moisture uptake and lower equilibrium values than the untreated controls. These findings are consistent with partial occupation of pores and lumens by a hydrophobic oil phase and reduced accessibility of water-binding sites. The results demonstrate that waste engine oil impregnation can substantially reduce hygroscopic response in the selected hardwoods. However, environmental safety, contaminant leaching, long-term weathering and application-specific durability require evaluation before engineering deployment.