In next generation wireless networks (NGWNs), interference is one of the most important considerations in terms of system design, deployment, and performance. Given the ability to use the spectrum in finer granularity such as in orthogonal frequency division multiple access (OFDMA) systems, scheduling seems to be one of the most important functionalities of NGWNs. Therefore, in this study, sub-carrier collisions for especially cell-edge users are investigated along with their impact on air interface capacity for OFDMA-based systems. It is shown that sub-carrier collisions follow hypergeometric distribution with different statistical characteristics. In addition, the air interface capacity of OFDMA systems with the availability of interference is studied as well. Numerical results are presented to verify the theoretical analysis and investigate the impacts of different parameters on both sub-carrier collisions and the air interface capacity.