Hello Secondary,
I manage the OANDA forums and came across your question. As stated in the OANDA website:
The United States Internal Revenue Service (IRS) requires non-US persons or corporations to complete a W-8 form to certify their non-American status in order for them to claim tax treaty benefits. Without this form, OANDA would need to withhold US taxes for the interest it pays on your account balance.
I am not aware of any minimum capital requirement applying with regards to this form. Let me know if this clear things up
