In respect of peer review Open Access journals are no different to subscription journals, in that different publishers and individual journals employ different styles & levels of rigour in their peer review processes.
Authors wishing to publish in any journal, Open Access or otherwise, should always check that the Journal has a peer review process that they are happy with.
Repositories will contain a mixture of pre-prints, post-prints and a smaller number of publisher pdfs depending on the policies of the publisher and the repository. This may result in some containing a number of versions of the same article, they will provide a link to the version of record where ever possible.
