Over the past 10 or so years, the number of [AIX, HPUX, Solaris] to Linux migrations have been increasing steadily, along with the "safe" recommended maximum number of concurrent users. The reason is simple, of course: cost. Unless your OpenEdge application is serving thousands of users, Linux can probably do the job just as well as one of the proprietary Unix flavours. If you have less than 250 concurrent users then this should be a no-brainer for 98% of you.
Here are some points to consider for the OpenEdge migration: