‘Roman’ essentially means plain text, the style you are viewing now; no bold, underlining or italics. You can have roman or plain text in any font, i.e. Calibri, Arial or Times New Roman (the use of Roman here is just part of the name which has been ascribed to the name of this font). See below for some examples :
Roman / plain text using Calibri font size 12
Roman / plain text using Times New Roman font size 12
Italics using Calibri font size 12
Italics using Times New Roman font size 12
You should pick one font style, either Calibri or Arial or Times New Roman and use throughout your work, therefore applying it to all text. Refer to OSCOLA oscola_4th_edn_hart_2012.pdf (ox.ac.uk) for guidance on where you should use plain text (roman) and italics in your work.
