I'm here to share a short description of the tax codes you mentioned above.
0%ME - Imports under the special scheme with no GST incurred (e.g. Major Exporter Scheme, 3rd Party Logistics Scheme).
0%OP -Purchase transactions that are out of the scope of GST legislation (e.g. purchase of goods overseas).
0%EP - Purchases exempted from GST (e.g. purchase of residential property or financial services).
As I've mentioned earlier, the descriptions are short and don't go into detail. If you need to know more of these tax codes, it's best to contact the IRAS, as well as consulting your accountant to know which tax codes work best for your company.
If you have other questions, feel free to post them below. I'll get back to you as soon as I can.