There are multiple ways to export midi files in Studio One...
Reference Manual: Editing > Exporting Audio and MIDI Files > Exporting MIDI Files
To export an Instrument Part to a MIDI file, [Right]/[Ctrl]-click on the Part and select Export Selection Parts to MIDI File. Choose a file name and storage location in the pop-up window, and choose a file type (Musicloop or Standard MIDI file). Then click OK or Save to export. A Standard MIDI File is created, with a .mid file extension.
Multiple Instrument Parts can be exported at once to a single file. To do this, select the desired Parts and use the same process as above, choosing the Standard MIDI option in the save window. One MIDI file is created that includes individual MIDI clips for each Part, in the correct sequence, with a common start time.
You can also drag-and-drop any Instrument Part to a location in the File Browser to export a Musicloop or MIDI file to that location. As you hover over the Browser, you can press [Alt] to choose between Musicloop and MIDI file formats. If you drag-and-drop multiple parts at once, they are written to one MIDI file with multiple Parts.
Standard MIDI Files exported from Studio One can be used by virtually any application that supports MIDI.
You can also use File > Save As and choose *.mid or *.midi as the format to export all instrument data to a MIDI file.