Oxygen in silicon is a problem well suited to modelling using AIMPRO. Oxygen has a rapidly varying potential close to its atomic core which is much more suited to fitting with real space Gaussian functions than sets of plane waves. Oxygen related point defects in silicon exhibit a variety of different defect symmetries which AIMPRO is able to constrain accurately. Our use of additional fitting functions placed at bond centred sites means that d orbital fitting functions are not required on the Si atoms, greatly simplifying the computational problem. Finally, FTIR has proved to be one of the most important experimental tools used to study these problems, and we are able to theoretically calculate both the vibrational modes and induced dipole moments of these modes. In combination with isotopic shift data these are a good way of comparing experimental data with the theoretical models.