The Saudi Army put down an Iranian backed Shia rebellion in Bahrain.
And it has at least achieved a stalemate in Oman against another Iranian backed Shia rebel group when the Oman Sunni government collapsed.
The Saudis have ~3k men in their Special Forces in an Army of ~175k.
Which is not enough to fight in Oman,put down any Shia rebellion in the Northeast of their country,and fight effectively in Syria.
But it seems the Saudis are expanding and upgrading their Special Forces despite $30/barrel oil and cuts in the welfare state.
Oman is neither Sunni nor Shia. They are majority Ibadi, which is a distinct sect. Iran never backed a rebel group in Oman. In fact, Iran and Oman have very good relations. You must either be thinking of another country or imagining things.