As with many of the questions regarding the Middle East on /r/askhistorians, I'm afraid I have to really question the premise of the question before I can give you a more detailed answer.

Oman was in a civil war known as the Dhofar Rebellion from 1962 to 1978, where the northern coastal region near Muscat was controlled by the British/Iranian-backed House of al-Said (leading to some really interesting propaganda posters like this being produced), and the interior of the country was controlled by various leftist and communist rebels. Before that, the interior of the country was controlled by a religious government called the Immamate of Oman until the 1950s, which, to put it diplomatically, was not on friendly terms with the al-Said dynasty. The country has been at peace for less than 50 years after decades of rebellions in the modern era, which to me does not make a state a "paragon of stability".

To expand on this answer, I would need you to clarify what you mean by stability and tolerance. Are you talking about their role as a diplomatic intermediary between the al-Sauds and Iran? Their non involvement in the Yemen Civil War? Also, I would always question the word "tolerance" because it's a very broad term that has very different meanings depending on who you ask. For example, was the Pact of Umar a mark of religious tolerance in that it allowed Jews and Christians to practice their religion, or religious repression because it privileged one religion over others? Different people will have different answers.