It's wise to have a CD/DVD ripper installed on your Mac just in case one day you want to make a copy of your disks. OSEx may now be discontinued and outpaced by smarter and more modern rippers, however it should get the job done for you.
Most users will unfortunately be put off by the rather unintuitive interface. If it's your first time working with the program you'll just have to click around and see what each button does, as OSEx doesn't come with any help file.
OSEx allows you to select from program streams, element streams, DVD folders, DVD images and Quicktime files. You can then select segments you want to rip (title, chapter, cell or VOBU) or simply rip the whole disc.
The time codes at the left of the interface will give you a sense of how much has been ripped. OSEx also lets you select individually sound, audio and subtitles to rip.
The application is generally fairly fast and shouldn't interfere with any other program running on your Mac.
I only really suggest installing OSEx if you want a very very basic ripper.