You could also check the option of multi level BOM, where the semi-finished product has a BOM which does not change, while the final product would he component + BOM 

https://www.odoo.com/documentation/15.0/applications/inventory_and_mrp/manufacturing/management/sub_assemblies.html

Hi, this can be done in several ways. I have post this before, here: https://www.odoo.com/forum/help-1/manufacturing-order-customized-as-per-the-customers-order-200517

and here: https://www.odoo.com/forum/help-1/how-to-make-the-addition-of-a-product-on-the-quotation-remove-other-products-from-a-bom-200655#answer-200695

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It depends on how complex the OEM customization is.

If there are set options for the OEM customisation (like making a sandwich at Subway) then you can use product variants and have a master BoM that contains all of the possible operations/components. On the BoM, you can select which operations apply for which variants.

For example, using the Subway example, the 'Lettuce' component and 'Add Lettuce' operation are only added to the Manufacturing Order if the variant has 'Lettuce' as an attribute. You can learn more here: https://www.odoo.com/documentation/15.0/applications/inventory_and_mrp/manufacturing/management/product_variants.html

If the OEM customization is more complicated and allows for custom attributes (Custom requirements, not selecting options), then you should use BoM templates. The BoM template should have all the components/operations that are common to each order. When an order for OEM comes in you can save time by duplicating the BoM template and adding the custom components/operations
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I hope this helps