Obelia longissima occurs primarily in the subtidal but occurs occasionally in the littoral if washed up or in rockpools (Cornelius, 1995b). Zamponi et al. (1998) reported Obelia longissima in the sublittoral of Argentina between 36 and 70 m depth. Stepanjants (1998) noted that Obelia species were found in all oceans, preferentially no deeper than 200 m but cited a record of Obelia longissima between 300 and 510 m deep in Patagonian waters.

Habitat preferences
Water movement is important for hydroids to supply adequate food, gas exchange, remove waste products, prevent excessive siltation and provide suitable substratum. Hydroids tends to be abundant where water movement is sufficient to but not high enough to cause damage. Hydroids with long stems tend to occur in calmer waters (Riedl, 1971; Hiscock, 1983; Gili & Hughes, 1995). Hydroids tend to occur in low light conditions, possibly due reduced competition from algae and/or settlement preferences of their planulae larvae (Gili & Hughes, 1995). The majority of hydroid species are stenohaline, i.e. do not tolerate reduced salinities. However, Obelia longissima was reported from sites subject to reduced salinity such as the Taw and Fal estuaries (JNCC, 1999). Temperature is an important factor controlling growth and reproduction in hydroids, and many species have optimal temperature ranges for reproduction (Gili & Hughes, 1995). For example, Berrill (1949a) reported that growth in Obelia longissima ceased at 27 °C and that newly formed hydranths rapidly regressed at 25 °C.