Installing the OAuth Packages

The OAuth extension requires both PEAR (which enables you to install the package) and libpcre3-dev, which enables the OAuth package to be compiled. To install the packages and confirm that OAuth is enabled, enter the following commands in the order shown:

apt-get install php-pear
apt-get install libpcre3-dev
pecl install oauth

If the following displays, you must edit your php.ini file to find the OAuth library:

    configuration option “php_ini” is not set to php.ini location
    You should add “extension=oauth.so” to php.ini

Open php.ini in a text editor (location in /etc/php5/apache2/) and add the following anywhere in php.ini:

    [OAuth]
    extension=oauth.so

Save your changes to php.ini and restart the Apache web server.
Now check again if that OAuth is Installed Successfully.

http://ralph.soika.com/magento-rest-api/