It seems *.spo output files from SPSS version 15 or earlier are only supported by this program of the specific version and no other 3rd party viewer or app is capable of exporting the data to anything else. SPSS supports a broad range of export options including Excel tables or images, however *.spo files cannot be opened in latest versions of SPSS.
Convert SPSS 15 and earlier output data to Microsoft Excel 97 to 2003 workbook.
We have found one software suitable in our database for this conversion.
Multiplatform software - convert spo to xls on any platform
A statistic and analytic software from IBM
Convert from spo file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.