My DSpace install is working fine, the metadata is being stored and showed correctly in any browser. The database has been confirmed as UTF-8. The problem is that the oai protocol shows accents and diacritics in letters (áéíóúüUñÑ etc) as ? like Dise?o instead of Diseño and all the entities that harvest our metadata report this problem. If you would like to see for yourselves, this is the link: http://repositorio.puce.edu.ec/oai/request?verb=Identify
I can't find any file that sets the encoding for the oai protocol nor any kind of solution for this problem.
Based on this thread: http://dspace.2283337.n4.nabble.com/OAI-tp4681419.html, you have to set -Dfile.encoding=UTF-8 in JAVA_OPTS. Then do a clean and force a rebuild of your OAI index as @terrywb mentioned (ie bin/dspace oai clean-cache and bin/dspace oai import -c -o.