The aim of this article was to develop a model for the usage of Open Access Institutional Repositories (OAIR) in university libraries in Ghana. The article adopted the Technology Acceptance Model (TAM), which was then modified to fit the study. The variables were adapted to improve the fit between data and the theoretical model, keeping the characteristics of TAM. Accessibility, availability and visibility were proposed in addition to the conventional variables of TAM. Positivist paradigm, quantitative methods research and survey design were used for the study. Simple random sampling and stratified random sampling were the sampling procedures and methods employed. A total of 998 respondents completed the questionnaires distributed. The questionnaire was used as a research instrument to gather relevant data for the study. Descriptive statistics (frequencies, percentages, means and standard deviation) and inferential statistics (multinomial logistic regression and Confirmatory Factor Analysis [CFA], using structural equation modelling [SEM]) were used as statistical tools to analyse the data. The study developed an OAIR Usage Model, which would be instrumental in the usage of OAIR in university libraries in Ghana. The model will enhance both user satisfaction and intention to reuse the OAIR, and make OAIR research outputs available, accessible and visible. This article adds to the body of literature on OAIR, as it is used in university libraries.