No, you will not use an O ring with our Small or Big Block Chevy engines.
The block is not machined for an O ring that some distributors come with. Make sure to use the proper distributor gasket when installing your distributor. Please follow instructions below for proper distributor installation:
https://knowledgebase.blueprintengines.com/knowledge/how-to-install-a-distributor-and-get-timing-set