Obviously we have a less than appealing newer Oscar puppet, but Caroll has said years ago that Oscar was one of the only Muppets that had never been replaced with a new build, and that the puppet just always went in for repairs and patch work, since aging helped with Oscar's overall appearance. Of course, he's also mentioned times where his eyebrow mech would break, resulting in only one half of the eyebrow being able to move up and down (ala
), which Caroll said he found amusing, but it gave Oscar some rather interesting facial expressions that kind of made him look mad or insane.
Otherwise, the SSU book mentions that certain Muppets tend to be rebuilt and replaced every five or so years, while certain others like Ernie and Bert, Grover, Cookie Monster, etc. were usually rebuilt and replaced every ten to twelve years.