For the R source and detailed documentation of the package,
download the R tar file, currently OOP_0.5-1_R.tar.gz.
Untar this, creating a directory OOP.
and run
R INSTALL OOPThe S-Plus version is temporarily not available. It will return.
Primarily the online documentation in the R package. See also a brief overview document and pointers from there. A paper appeared in the September, 2001 issue of the R Newsletter, introducing OOP programming for the S language (specifically for R, but the OOP package applies to both R and S-Plus). It's a little out of date in details, but gives some background that may be helpful.