Olive marries the Prince and becomes a princess.
Excellent! The best marriage there is for Olive.
Sin: Less than 100
Olive must visit the "Young Officer" at the palace every January without missing a single year, unless she has a birthday on January 31st.
Tips And Tricks
It is important to make sure Olive does not miss a single year of talking to the Young Officer - and she must promise to meet with him again the next year. A good way to remember to meet with the Young Officer is to set Olive's birthday to the 31st of January - to serve as a reminder to meet with him that day. This also gives you an extra month to either recover lost relationship points, or to talk to someone else. It is also important to raise Olive's Charisma and Refinement during the 8 years of caring for her. Charisma needs to be atleast 200 and Refinement needs to be atleast 250. Her Sin should be less than 100, so remember to reduce her Sin before the game ends.
You shouldn't risk Olive falling "in love", since each time it occurs her relationship with the Prince, the father and Cube all drop by 6%.She needs her relationship to be atleast 90% to marry the Prince, but in most games it can only be raised upto 96% since she gets only 8 meetings, each of which raise her relationship by 12%.
Note that you can get this with the Queen Ending, where Olive herself is appointed as the future Queen of the Realm. As long as Olive meets the requirements for both of them she will become the new heir to the throne AND marry her predecessor, killing two birds with a stone.