Serious Issue ....... that MUST be Addressed !!!!!
I usually use the Oxford comma, but sometimes the sentence has the same meaning and flows better without it. For all commas, I consider how the sentence should be spoken. When a pause before the "and" preceding the final listed item is appropriate, I include the comma. Where the intent is conveyed better as a continuous stream, I omit it.
Paul's letters may be the ultimate reference on commas and full stops: use his run-on writing style as an example of what to avoid!
there, their, they're Simon