Because positive streamers propagate opposite to the electron drift velocity, their growth not only depends on the local electric field, but also on the electron density ahead of them. We have recently demonstrated the importance of this electron density, by showing that positive streamers could be guided by weak preionization from a laser. Here, we study streamer guiding in nitrogen–oxygen mixtures in more detail, to better understand the propagation of positive streamers. First, the conditions required for guiding are investigated, both experimentally and with three-dimensional numerical simulations. Particular focus is on the role of oxygen and photoionization, which tend to inhibit guiding. Then the position of the guided streamers is investigated, and successfully related to the electron drift.