There is no preferred order.
However, you must ensure that SharePlex and Oracle versions are compatible.
So if you decide to upgrade Oracle first, you need to ensure that the version of SharePlex currently installed is compatible and supported on the new version of Oracle.
If not, you may be required to upgrade SharePlex first, again ensuring that the new version is compatible and supported on the existing version of Oracle.
It is worth testing out the procedure in a QA environment first.