How can the OR selector be used with labels in Kubernetes?

Now you can do that :

kubectl get pods -l 'environment in (production, qa)'

https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#list-and-watch-filtering

OR isn't supported in your case.

You can try the set-based label selector. Refer to the following link, https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#resources-that-support-set-based-requirements