You can legally use it once you own it. You can only legally BUY it in combination with 'qualifying' hardware -- a CPU, hard drive, graphic card or keyboard/mouse combination (see microsoft.com for the accurate terms and conditions.)
The seller is only allowed to sell it to you along with hardware, that;s what makes it OEM (or 'bundled') You also get less support from Microsoft (because its supposed to be provided by your OEM vendor, why its cheaper) and many OEM versions do not qualify for reduced cost upgrading to newer versions in the future (again, details will vary)