Download and install a command-line audio conversion program that handles both OGG and MP3 formats. One of the best is SoX, a free, open-source program for Linux, Mac and Windows. Other conversion programs are available, including dBpoweramp and Media Monkey, though most of these work with a specific operating system and may cost money.
Open your operating system's command line interface. In Windows, this is Command Prompt, found under Accessories in the Start menu. Mac calls it Terminal, found under the Utilities folder. Linux distributions may call it a terminal or shell; it's usually found under the Accessories tab in most graphical interfaces.
Type the command to convert the file and press "Enter." The command is usually the name of the program, followed by the OGG filename, then the MP3 filename. Consult your program's documentation if unsure. In SoX, the command looks like this:
sox file.ogg file.mp3
If your import and export files will be in different directories, enter the complete path.