One would probably have to define "one-man show". But, no, Theo does not write most of OpenBSD's code. Although he certainly is the leader of the project. If the copyright statement bothers you, I can recommend the announcement e-mail (see "THANKS" in particular):

https://marc.info/?l=openbsd-announce&m=171228270018970&w=2

You will also find separate copyright statements in a lot of the source files and many carry the name of their authors.