Nope, nothing on Oman specifically. (Wait, is this the reputation I have now?! )
From some quick research, you’ve got HSBC there so you’re in with a decent shot at ATMs working. Posts on Tripadvisor’s forums seem to indicate that there are a variety of different ATMs in all populated areas, some that will charge a fee. Mastercard’s own ATM finder seems to recommend HSBC as the major brand with chip reader support though.
Be aware of the general tips for travailing with Monzo as well as the daily, rolling 30 day and annual limits on withdrawals (£250, £1,000, £3,000), take enough cash to last you the first few days along with at least one alternative card (we are in beta, remember) and you should be fine! Let us know your experiences when you get back.