The flag of Oman (علم عُمان) consists of a horizontal tricolor of white, red and green with a vertical red stripe on the left that contains the national emblem of Oman (Khanjar and two swords). (Source: Wikipedia)
You need:
- Red banners (6x Red wool + a stick)
 - A loom (2 string and 2 planks)
 - 2x Red Dye
 - 2x White Dye
 - 1x Green Dye
 - 1x Shield (Bedrock Edition from Tails and Trails update)
 
How to make it:
- Make a loom. Put two strings and two planks together in the crafting table.
 - Make red banners using one stick and six blue wool in the crafting table.
 - Bring the red banner into the loom.
 - Add white dye and pick the Pale Sinister design.
 - Add green dye and pick the Pale Dexter design.
 - Add red dye and pick the Chief design.
 - Add white dye and pick the Chief Intented design.
 - Add red dye and pick the Chief Dexter Canton design.
 
That’s the finished Oman flag as a banner all done! So now we want to put the banner on our shield, but first we want a couple more of these banners so we can decorate the village with them.
- Put the finished banner in the crafting table. Add more red banners next to it. That’s how you copy or duplicate the banners.
 - Now put one of these banners next to a shield in the crafting table and now you have a Oman shield! This also works in Bedrock edition from the Tails and Trails update!