pycsw supports the The Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) standard.
OAI-PMH OpenSearch support is enabled by default. HTTP requests must be specified with mode=oaipmh in the base URL for OAI-PMH requests, e.g.:
See http://www.openarchives.org/OAI/openarchivesprotocol.html for more information on OAI-PMH as well as request / reponse examples.