E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
O. P. Riot (Old Price Riot).
When the new Covent Garden theatre was opened in 1809, the charges of admission were increased; but night after night for three months a throng crowded the pit, shouting O. P. (old prices); much damage was done, and the manager was obliged at last to give way.