An oud carries its strings in courses, most of them paired, and usually a single bass string alongside them. The upper courses are plain nylon or a similar synthetic; the lower ones are wound, with the winding doing the work of adding mass without adding stiffness. Sets are sold by tradition and by pitch, which is the first thing to get right.
Buying a set
Arabic and Turkish instruments are usually strung to different pitches, and sets are sold accordingly. Putting a set intended for a longer Arabic scale onto a shorter Turkish instrument, or the reverse, gives you either a slack, unfocused sound or more tension than the top was built for. Match the set to the instrument rather than to the price, and if a seller cannot say which tradition a set is for, that is an answer.
The wound courses
Wound strings die from the inside. Sweat and dust work into the winding, the mass distribution stops being even, and the string goes both dull and slightly false — it stops playing perfectly in tune with itself along the fingerboard. On an instrument with no marked positions that shows up as your intonation mysteriously getting worse, which is why an oud can feel like it has stopped cooperating when it simply needs strings.
The risha changes as much as the strings
The risha is a long, flexible plectrum, traditionally a feather quill and now more often plastic or horn. Its flexibility, width and how far you let it protrude from the hand all change the attack more than a string brand will. A stiffer risha gives a harder, more percussive attack; a softer one gives a rounder sound and more control at speed. Try several — they cost almost nothing and they are the cheapest way to change how an oud sounds.
Changing them
Change a course together, and change the whole set when the wound strings go. On friction pegs, string one course at a time and keep tension on the bridge — a floating bridge that comes loose because every string was slackened at once turns a ten-minute job into a difficult afternoon.