Open Access (OA) generally and traditionally refers to peer-reviewed research journal articles that are freely available with no access barriers. More information about Open Access can be found on the SPARC website
Open Access (OA) may also refer more broadly to freely available materials that are outside the research journal article OA definition and may include images, films, data, books, open source, etc. that are freely available with no or few access barriers.
Open Access Week is typically held the last week of October. See more information here - Open Access Week