Rosetta Code's programming chrestomathy site for Rexx. has many examples under categories ooRexx and REXX. Many of those under REXX need changes if you want to run them with ooRexx. The author uses some language elements that are not supported by ooRexx: @#$ as/in variable names, an assignment without righthand side (s=;), and the Upper instruction.
Vladimir Zabrodsky's Album of Algorithms and Techniques. has a wealth of mathematical and other algorithms.
Walter Pachl's rxm.cls contains an ooRexx implementation of the rxMath package providing function results to very large precision.