Guys...is WD caviar red any good---durability and performance wise?

OEM are those batches that WDC made for other system builder or distributors such as Dell, HP, etc. that includes WDC hard disk with their system.
Those OEM hard disk come with cheaper price and of course you can't deal with WDC directly if there's any defect, you will need to refer to the system builders or distributors themselves for claiming warranty.
I'll prefer the retail version though, you can check if it's the retail unit or OEM unit by checking their warranty status on the spot if you have a smart phone with you (provided you can online).

For those external drives, they're packed with proper plastic or good looking boxes but not for those internal 3.5" drives that I have seen.
Best I have seen is Samsung internal drives and it also includes some screws.
Probably for the sake of cutting down the cost of producing the super-good-looking boxes or included accessories, that's why the internal drives that we bought usually only has simple static bag or just plastic shell (Samsung).
Either way still can check if it's under OEM or not, Seagate or WDC RMA check is good way to determine that.

At my local retails, the only and possible options is WDC, Seagate is over-sell, not sure why, the price different does not really justify it so I went for WDC (oh well.) but goes for the AV-GP only if available, 1 more year only cost RM20~30 more depends on supplier.

As for getting the OEM drives, erm... wonder if the seller is willing to give you the serial numbers for you to check its details at WDC support, just to make sure it's really OEM unit, and if it's OEM then you need to refer back to seller if there is any problem, quite risky I would say, you never know when will it fail and the seller already not more to be found by then.
.....

just buy any brand / model?