This paper provides a general overview of the methodology implemented in ONETEP (Order-N Electronic Total Energy Package), a parallel density-functional theory code for large-scale first-principles quantum-mechanical calculations. The distinctive features of ONETEP are linear-scaling in both computational effort and resources, obtained by making well-controlled approximations which enable simulations to be performed with plane-wave accuracy. Titanium dioxide clusters of increasing size designed to mimic surfaces are studied to demonstrate the accuracy and scaling of ONETEP.