Welcome to Cruise Critic!
I am sorry your Dad is so poorly, but I am right there with you. Last fall, I obtained Holland America's Standard Cancel For Any Reason (CFAR) cancellation protection for a 35-day cruise that would have sailed in mid-February. There is no medical included with this plan, so pre-existing conditions had no bearing. The closer the cruise got the hollower my stomach felt!
At the last possible moment (24-hours) I cancelled. The amount I calculated for refund was repaid to the penny within a week. Note, it being such a long cruise, with the protection premium and the amount withheld from refunding, it still cost us about $5,000 to cancel, but my family members' health did proceed to decline, and I was happy not to have been in the middle of the Pacific all last month!
"Worry for family members" is not covered in any plan, of course. Does Oceania not have CFAR cancellation plans? No? Maybe @iamtrustworthy knows of a third-party plan.
Good luck to you and your Dad.