Probably one of darker periods of Omani history was time period in which Oman and Omanis were heavily involved in the East African slave trade.

Arabs and Persians began settling and trading in East Africa as early as the 9th and 10th centuries. There's a debate about whether this early trade between al Khaleej and East Africa involved slaves, or other goods. However by the 19th century, the trade of slaves was an integral part of the Omani economy. Oman became involved in East Africa starting in the late 17th century. After parts of the East African coast came under Omani rule, many Omanis began settling in East Africa seeing the vast economic possibilities the newly acquired land could bring, building on already established trade links between East Africa, al Khaleej and India.

I'll specifically talk about the 19th century because that's the time period I'm most familiar with. During this time Zanzibar and Oman were dependent on each other, or to be more precise Oman was heavily dependent on Zanzibar for its financial success. "Oman," if we can really even call it that was heavily bifurcated between "Oman" and "Zanzibar." Oman is what we now know of as the country on the Arabian Peninsula, and Zanzibar was not only the island we know today, but also coastal areas controlled by Omanis that stretched from Kenya to Mozambique and various islands in the Indian Ocean. For a while the capital and royal family of Oman was even moved from Muscat to Stone Town on Zanzibar island owing to its importance. Zanzibar and Oman split politically in 1856 but still remained very much intertwined with each other.

Oman was relatively prosperous during this time. Almost all wealth was being created from activities (including the slave trade) originating in East Africa. One particular aspect of this wealth was that clove (and other) plantations were developed on Zanzibar to feed into the global market for cloves. These plantations created a high demand for slave labor. In Oman and most of the rest of the Middle East, slaves primarily served small scale less labor intensive domestic roles. However in Zanzibar, much more labor was needed for the plantations and related agricultural activities.

Slaves were taken from central Africa and marched usually in chains, to the coast. Slave traders penetrated further and further into the continent to capture slaves. This was often violent and extremely disruptive for communities in the interior. Whole villages were often enslaved or killed by these slave traders. Tippu Tip is probably the most well-known Omani slave trader. The march to the coast was brutal and an unknown number of people died along the way. Once they reached the coast they were sold locally, or shipped to Oman or other parts of al Khaleej. The number of slaves brought to the coast in the mid-19th century likely totals at least 200,000.

The demand for slaves was less in Oman since there were few large scale agricultural establishments requiring slave labor. Between the 1830's and 1860's approximately 4,000 to 10,000 slaves from East Africa were shipped to Oman every year. Sur in Shargiya was a major slave port, and likely the primary slave port for the rest of the country. Slaves were landed in Al Aygah in Sur then divided up to be sent to the rest of the country. Muscat also had an active slave market.

By the last half of the 19th century, pressure by European powers was mounting to stop the slave trade. The British began patrolling the Indian Ocean for slave trading vessels and attempting to "free" the slaves (although it should be noted here the British's intentions weren't always altruistic). The sultans of Zanzibar realized that the slave trade would face greater and greater international opposition. This culminated with the Sultanate of Zanzibar signing a treaty with the British in 1876 to essentially stop the slave trade. The treaty ended the slave trade, but allowed Arabs/Omanis to continue to keep slaves. The illegal slave trade continued until about 1902. Zanzibar abolished slavery in 1897 and Oman abolished slavery in 1962.

Sources:

Oman Since 1856 by Robert Geran Landen

Oman a History by Wendell Phillips

Narrative of a Year's Journey Through Central and Eastern Arabia by Gifford Palgrave

Dhow chasing in Zanzibar waters and on the eastern coast of Africa. Narrative of five years' experiences in the suppression of the slave trade by George Lydiard Sulivan

Memoirs of an Arabian Princess by Emily Ruete

Tropical_chancer has given a great answer regarding slavery and Oman and I hope others can add to it. I recommend reading the works of Jerry Zdanowski, who has written at length about slavery in the Gulf - here’s one of his manuscripts: https://repozytorium.ka.edu.pl/bitstream/handle/11315/6923/Zdanowski-Slavery.pdf?sequence=1 - see page 20 to read how profitable the trade in slaves was for Oman.

I wanted to touch on something more meta though, which is the learning history in the Gulf. First, you are right that in our region history as taught in our curriculums and by the state tends to whitewash (this is not exceptional to the Gulf and happens in countries like Britain too, but state control of publications makes it an acutely felt phenomenon). History tends to be taught in a manner that amplifies the prestige of the state. For example, the only parts of Bahraini history that is taught from the period before the Al Khalifa took the islands are the ancient history, early Islamic history, and the Portuguese occupation. The parts inbetween - like when Bahrain was part of the radical Qarmatian republic, or its rule under medieval Shia dynasties, or its century as part of the Safavid Persian empire - aren’t covered. Nor are the conflicts between Qatar and Bahrain, occupation by the Saudis or multiple wars/occupations by Oman. The reason comes down to politics, which determine how and why national history is written. In the case of the Gulf, it isn’t just the prestige of the state but also of the dynasty. So it is not useful to dwell on the wars with today’s GCC allies, and while empire might be celebrated, slavery maybe whitewashed from the picture.

This presents a challenge to accessing history, especially if you’re not in a university with access to the (quite active) academia on our region’s history. One place I can recommend is the Qatar Digital Library, which has several easy to read articles on Omani history: https://www.qdl.qa/en/articles-from-our-experts?places_in_the_arab_world%5B0%5D=639

Despite the name (the library is funded by Qatar) the articles and, really, everything, is the product of academics working in the British Library, and it isn’t partisan (however, all historians have some bias - use your critical thinking as you read to come to your own judgements). The QDL is also in the process of digitising the British colonial archives (the India Office Records, covering records from the mid 1700s to 1948). You may be interested to read these directly, but if you do just keep in mind they are the writings of colonial bureaucrats who were trying to understand the Gulf in order to control and dominate it for political and economic gain - so they are hardly unbiased sources.

I’d encourage you to start - and keep reading.