Only-Search is considered a Potentially Unwanted Program (PuP) that usually gets installed by having been being bundled with some freeware program that a user chooses to install. It can hijack and redirect a browser homepage, search engine, and/or certain browser settings (to allow cross-site scripting for its own advantages), as well as can alter various existing program shortcuts on a system, pointing them toward its own web page. The goal of all this is to gain click-revenue from ads and sponsored links thereon for its makers.
Depending on how it was installed and the malware barriers then present in your browser and system, it may have only altered a few things in the browser all the way up to burying itself very deeply into the operating system of the computer. Typically Only-Search's browser extension/plug-in has been known to afflict IE, Firefox, or Chrome-based browsers.
Adequate removal methods for the program depend on the depth of infection. Usually, it's best to start with using removal tools like both AdwWare Cleaner and Malwarebytes (and in stubborn cases, even a third tool like Hitman Pro) to get rid of anything related to Only-Search from the registry and program files. Then a user needs to check each of his desktop, taskbar, and start-page program shortcuts to make sure that the "hxxp://www.Only-Search.com" term (here obfuscated from "http" to keep the link unclickable) has not been appended to each/any shortcut's executable and start paths... if it has, delete just that term and save the shortcut. Also, after all this, it's still wise to make sure Only-Search isn't present in the browser's home-page/speed-dial settings nor in its list of plug-ins/extensions. Finally, verify that any browser cross-site scripting 'prevention' settings haven't been altered.
If all else fails or you're shy about trying this on your own, you should proceed to a reputable security clean-up site like DSLR Security Cleanup, Wilders, etc to have a security guru guide you through the process.