OmpA can form folded and unfolded oligomers

► OmpA refolded from urea close to the cmc forms oligomeric and monomeric species. ► The oligomers exist both in the folded and the unfolded state. ► Folded and unfolded oligomers can interconvert. ► We propose that folding occurs by β-strand mediated domain swapping. ► This occurs in lipids as well, highlighting the need for chaperoning activities.