I don't use OpenOffice for several reasons -- it just doesn't/didn't do sometimes what I want to do.

But here might be the answer to your question: https://wiki.openoffice.org/wiki/Documentation/OOoAuthors_User_Manual/Writer_Guide/Using_word_completion


Desktop; i7-2600k, 8 GB mem., Win 10 Pro, BIAB 2017; RB 2017 - latest build
Laptop: i5-2410M, 4 GB mem, Win 10 Pro, BIAB 2017; RB 2017 - latest build