Step 1: Opening Balance Sheet.
When a not-for-profit organisation is formed, its opening Balance Sheet is prepared to show its initial financial position.
Step 2: Source of data.
The Receipts & Payments A/c provides information of cash and bank balances, assets, and liabilities, which are used to prepare the opening Balance Sheet.
Step 3: Conclude.
Thus, the opening Balance Sheet is prepared from Receipts & Payments A/c.
Final Answer:
\[
\boxed{\text{Receipts & Payments A/c}}
\]