I installed win 8.1 OEM on new built PC but after 2 weeks, I took it apart and returned it. I was wondering if can reuse OEM if I buy same motherboard later?
Yes. Windows 8 and 8.1 do not have the same motherboard restriction as earlier version of Windows.
From the license agreement:
MICROSOFT SOFTWARE LICENSE AGREEMENT
"Can I transfer the software to another computer or user?You may transfer the software to another computer that belongs to you. You may also transfer the software (together with the license) to a computer owned by someone else if a) you are the first licensed user of the software and b) the new user agrees to the terms of this agreement. To make that transfer, you must transfer the original media, the certificate of authenticity, the product key and the proof of purchase directly to that other person, without retaining any copies of the software. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Anytime you transfer the software to a new computer, you must remove the software from the prior computer. You may not transfer the software to share licenses between computers. You may transfer Get Genuine Windows software, Pro Pack or Media Center Pack software only together with the licensed computer."
As long as it is on only one PC at a time, you are fine.