Usually, OEM cards have less features and/or less performance, little/no warranty, less bundled software (usually just no bundled games), and sometimes fewer connectors/adapters.
For example, I have an ATI OEM 8500. It's clocked at 250 core/275 memory (not bad for an OEM) compared to 275/275 for the retail version. DVD software and drivers were bundled but no other software (IIRC). An S-Video cable was included but I think the retail version R8500 included an S-Video to Composite adapter or included a multifunction dongle. My OEM R8500 has VGA-out and DVI-out but the latter is not DVI-I, meaning the connector does not support a second CRT only a digital flat panel. There's no second RAMDAC anyway. For dual monitors I can have CRT + Flat Panel but not CRT + CRT. (I could have flat panel + flat panel but one would need an analog, VGA interface).
Not all OEM cards have identical features. (Some retail cards may have differences as well). Look carefully at what you are buying. Make sure it supports/includes what you need. Ask questions (of the vendor) if you are not sure. Don't assume anything.
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