What is the minimum amount the OR/SI is required to be issued by the taxpayer ?

As a rule, the minimum amount is 100.00.

Section 237 of our Taxation Code, as amended, provides that all persons subject to an internal revenue tax shall, at the point of each sale and transfer of merchandise or for services rendered valued at One hundred pesos (₱100) or more, issue duly registered receipts or sale or commercial invoices, showing the date of transaction, quantity, unit cost and description of merchandise or nature of service: Provided, however, That where the receipt is issued to cover payment made as rentals, commissions, compensation or fees, receipts or invoices shall be issued which shall show the name, business style, if any, and address of the purchaser, customer or client: Provided, further, That where the purchaser is a VAT-registered person, in addition to the information herein required, the invoice or receipt shall further show the Taxpayer Identification Number (TIN) of the purchaser.