I don't think it was ever literally said so by another character, but is often said that Oikawa went through a rather large alien obsession phase as a child because in all the flashbacks to to childhood he is seen wearing alien shirts or with other alien things. In one he also seemed to be watching some program on tv about them while holding a sign that said something alone the lines of "go aliens." So if wanted, it can stretched to say Oikawa also obsessed over space Just thought I'd help. . :)
Thank you! I hope you don't mind me publishing this because it's come up with several fandom friends and we've always sort of shrugged at each other and carried on without checking. This made my day; I'm so glad there is canon evidence for an alien-obsessed Oikawa.
