When you use SRename with a file manager like Directory Opus, and you have set it up to receive multiple filenames ({O} in DOpus), it's better to place that argument last, because if the command line buffer gets full then the arguments at the end of the command line will be ignored.
It's better that some filenames don't get renamed than the rename operations not to be executed at all. For example:

SRename UPPER {O} is recommended.
SRename {O} UPPER should be avoided.

It is also recommended to use the NOPAT option when using SRename with a file manager when you don't intend to use wildcards, because filenames that contain 'illegal' characters can trigger the use of wildcards accidentally.