It is often stated that you want to avoid oxygen exposure to your hard cider after fermentation begins. While this is a good practice, like most questions related to the production of hard cider, the answer often is it depends. With wine, micro-oxygenation can have positive effects on the characteristics of some wines. Micro-oxygenation can improve color in red wines and flavors, especially when aging with oak(1). This can be accomplish through traditional barrel aging or oak chips where oxygen is purposely infused. Even the transmission of oxygen through corks can have a positive impact. However, you must remember that acetic acid bacteria and Brettanomyces, two spoilage organisms for wine and hard cider, also favor oxygen enriched environments post fermentation.
Whether you favor some micro-levels of oxygen or want to avoid it, I found it interesting how various processes can induce oxygen into your hard cider. This seemed like the perfect Mālus Trivium topic.
(1) W. J. du Toit, Stellenbosch University, Micro-oxygenation, oak alternatives and added tannins and wine quality, Woodhead Publishing, 2010
(2) Wine Science, Fourth Edition, Post-Fermentation Treatments and Related Topics, Chapter 8
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Want more details about making and enjoying cider, check out these posts.
PricklyCider.com 
I can really recommend Postmodern Winemaking for those who want to do a deep dive into oxygens role in flavor development.
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