Depends on what you mean by "OEM".
True "OEM" Windows is what you get when you buy a pre-built machine (Dell, HP, Gateway, etc.). It's tied to the motherboard/CPU that the machine is sold with, so it's very difficult (previously impossible, but Microsoft has lightened up a bit) to switch it to a new motherboard/CPU.
When you buy a copy of Windows by itself (whether from a brick-and-mortar store or online from Microsoft or an authorized reseller), the retailer may label it as "OEM", but it's actually the 'retail' version.
The biggest difference, however, is in the bells & whistles. OEM versions tend to only come with the drivers that go with their particular CPU/motherboard combination, & 99% of the time have useless bloatware included. Retail versions tend to have a wider selection of drivers (since they don't know what combination of hardware you'll have), & don't have the bloatware included.