multi-select OR values to filter entries
Not an expert myself, but you can try .contains('OR', 'a', 'b', 'c', 'e')

However, rather than first fetch all entries and then filter them, I would look into using relations when looking them up, so that the initial result only contains the entries you're after.

Have a look at https://craftcms.com/docs/relations to get started, and don't be afraid to ask for help if you can't quite figure them out (I found them a little tricky myself when first attempting to use them).