What countries are in the OMX?

Could you please elaborate on the countries that are currently a part of the OMX? I understand that OMX is a stock market index, but I'm curious about the specific nations whose companies contribute to this index. Are there any notable economies or regions that are well-represented within the OMX? Additionally, is there a process for countries or their companies to be included or excluded from the OMX, and if so, what are the criteria for such decisions? Thank you for your clarification on this topic.