List of journal abbreviations prints abbreviation instead of long journal title

With a sourcemap you essentially manipulate your entry data just when it is read from the .bib file. This means that for all intents and purposes overwriting journaltitle with shortjournal with a sourcemap means that journaltitle now contains the contents of the shortjournal field and that the old field value cannot be recovered on the biblatex side.

One solution would be to save the original journaltitle in a different field (say longjournaltitle, we need to declare this new field in a datamodel file) and use that when required in the list of journal shorthands.

documentclass{article}

begin{filecontents}{longjournaltitle.dbx}
DeclareDatamodelFields[type=field, datatype=literal]{longjournaltitle}

DeclareDatamodelEntryfields[article,perioduical]{longjournaltitle}
end{filecontents}

usepackage[style=oscola, datamodel=longjournaltitle]{biblatex}

DeclareBibliographyDriver{shortjournal}{%
  printfield{longjournaltitle}}

DeclareSourcemap{%
  maps[datatype=bibtex]{
    map[overwrite]{
      step[fieldsource=shortjournal, final]
      step[fieldsource=journaltitle]
      step[fieldset=longjournaltitle, origfieldval]
      step[fieldsource=shortjournal]
      step[fieldset=journaltitle, origfieldval] 
    }
  }
}


begin{filecontents}{jobname.bib}
@article{Herlin2008aa,
  author       = {Herlin-Karnell, Ester},
  number       = {10},
  pages        = {1115 -- 1130},
  title        = {What Principles Drive (or Should Drive) European Criminal Law?},
  volume       = {11},
  date         = {2008},
  journaltitle = {Common Market Law Review},
  shortjournal = {CMLRev.},
}
end{filecontents}
addbibresource{jobname.bib}

begin{document}
nullvfill % just for the MWE
Here is a reference to an article with an abbreviated journal nameautocite{Herlin2008aa}

printbiblist[title={Journal Shorthands}]{shortjournal}
end{document}

Another solution would be not to remap the fields at all. Instead we instruct your style to print the shortjournal if available.

This is done by redefining the journal bibmacro from biblatex.def.

The sourcemap does not remap any fields any more, it just removes .s from the field (this is a copy of the sourcemap for journaltitle from oscola.bbx).

documentclass{article}
usepackage[style=oscola]{biblatex}

DeclareBibliographyDriver{shortjournal}{%
  printfield{journaltitle}}

DeclareSourcemap{%
  maps[datatype=bibtex]{
    map[overwrite]{
     step[fieldsource=shortjournal,
           match=regexp{(d).(d)},
           replace=regexp{$1.$2}]
     step[fieldsource=shortjournal,
           match=regexp{(D).(d)},
           replace=regexp{$1 $2}]
     step[fieldsource=shortjournal,
            match=regexp{.(D)},
            replace=regexp{$1}]
     step [fieldsource=shortjournal,
            match=regexp{(D).},
            replace=regexp{$1}]
    }
  }
}

renewbibmacro*{journal}{%
  iffieldundef{shortjournal}
    {ifboolexpr{
       test {iffieldundef{journaltitle}}
       and
       test {iffieldundef{journalsubtitle}}
     }
       {}
       {printtext[journaltitle]{%
          printfield[titlecase]{journaltitle}%
          setunit{subtitlepunct}%
          printfield[titlecase]{journalsubtitle}}%
        newunit}}
    {printtext[journaltitle]{printfield[titlecase]{shortjournal}}}%
  iffieldundef{journaltitleaddon}
    {}
    {printfield{journaltitleaddon}}}

begin{filecontents}{jobname.bib}
@article{Herlin2008aa,
  author       = {Herlin-Karnell, Ester},
  number       = {10},
  pages        = {1115 -- 1130},
  title        = {What Principles Drive (or Should Drive) European Criminal Law?},
  volume       = {11},
  date         = {2008},
  journaltitle = {Common Market Law Review},
  shortjournal = {CMLRev.},
}
end{filecontents}
addbibresource{jobname.bib}

begin{document}
nullvfill % just for the MWE
Here is a reference to an article with an abbreviated journal nameautocite{Herlin2008aa}

printbiblist[title={Journal Shorthands}]{shortjournal}
end{document}