The elve is the dominant type of transient luminous event (TLE) at the base of the E region ionosphere. The hydroxyl nightglow (OH* nightglow hereafter) has been reported at a similar altitude. Statistical studies show that the mean heights of elve and OH* nightglow are 87 km and 87.5 km, respectively, and that 91% of the 291 limb elves are located within ±5 km of the altitude of the brightest OH* nightglow emission and both elves and OH* nightglow show the semiannual oscillation (SAO) at low latitude. The physical reasons for the collocation of elves and OH* nightglow are the main focus of attention. A model for elve emission with the environmental-adapted electron density profile is tested with three density profiles of atomic oxygen (O): the original profile from the NRLMSISE-00 model and two other profiles which are shifted 5 km upward/downward from the original. For higher altitudes of the given O density profile, the peak altitudes of the elve emission layers are also increased. This result reveals the leading role of atomic oxygen in the collocation. Furthermore, the altitude variation of elves is compared with the VLF reflection height observed by DEMETER. The latter height not only shows the SAO similarity to the elve/OH* heights but also exhibits a difference between land and ocean. The possible relationship between VLF reflection height and elve/OH* height is also discussed.