O'Print is described as 'This is an amazing printing solution for iPad and iPhone. – No apps needed – Any printer works – Print from Dropbox and folders – Secure your printing'. There are four alternatives to O'Print for Mac and Windows. The best alternative is handyPrint, which is free. Other great apps like O'Print are Printopia (Paid), Mirage by DINAX (Paid) and Presto (Paid).
handyPrint, previously AirPrint Activator, is a small application that will enable AirPrint, a hidden feature to allow your shared printer to be visible on your iOS 4.2+ devices, in Mac OSX 10.5, 10.6 and 10.7.
Use AirPrint to print to your Mac's shared printers, from your iPhone or iPad. Run Printopia on your Mac to share its printers to your iPhone, iPad or iPod Touch. Add unlimited virtual printers to save print-outs to your Mac as PDF files.
Presto, previously FingerPrint, enables AirPrint on any printer. All of your printers are displayed in an easy to read list in FingerPrint's main window. You can then select which printers you want to enable for AirPrint.