A technique is developed for analysing the linear collapse properties of spatially linear two-dimensional null points with open boundary conditions. A treatment is given of the collapse of nulls which have current and flow so that they are initially in a steady-state balance between a magnetic force, a pressure force and a centrifugal force. This extends the previous results for initially current-free X-type nulls with no flow. It is found that all X-points, regardless of the current and flow tend to collapse. Also, O-points collapse in the absence of a plasma flow, but O-points with a large current and possessing a highly super-Alfvénic plasma flow can be stable against linear collapse.