In my field (Computer Vision), the largest international conference (CVPR) has the following format: two simultaneous oral sessions (with talks awarded to ~10% of accepted papers), followed by a single poster sessions (awarded to ~90% of accepted papers). I have recently had an oral presentation of my work at this conference, and I am now writing a cover letter for a job which will be reviewed by a non-specialist, who will not necessarily understand the prestige of such a talk.
So, rather than writing "I was awarded an oral presentation of my work", I'm wondering how fair it would be to say "I was awarded an oral / plenary presentation of my work", to indicate that the oral sessions are to half of the entire conference delegates. If I just wrote the first option, then it may be assumed that the oral sessions is just with a small group and among many simultaneous oral sessions, as is typical at many conferences.
So would it be correct to say "oral / plenary", or does this imply something even more prestigious (e.g. a keynote)?
No, your talk was not in a session that would even pass the definition of plenary session. More importantly, it sounds as if the conference organizers also did not name you as a plenary speaker.
Definition of plenary session: In a conferences running multiple simultaneous sessions, a plenary session will be a special session when only one session will be scheduled.(no other sessions scheduled in that slot). The plenary speaker(s) are usually quite senior or somehow big-name-draws, and may give a general, high level review talk(s). Rarely there may be only one plenary speaker with a few (only) invited speakers in support without the 'plenary' title, particularly if the organizers wish to not dilute the honor they are giving to a particular big name draw.
Common sense rule in CV or similar document is to use the exact nomenclature used by the conference program to describe your talk (This includes also not messing with the listed author order in pretense of making yourself look better). If the organizers did not distinguish your talk from the regular talks as being an 'invited', 'plenary', 'keynote', or 'review' presentation, then do not rename it yourself. (You can always add some sentence or phrase that explains some additional context why you think it is so much more important than it sounds, but usually, if the prestige of one presentation makes or breaks your document, then you have other problems!)
Not all multi-symposia conferences will choose to spend time to run plenary session(s), or they may not use the nomenclature for what seems like a plenary-look-alike session. However, if they do not, renaming it yourself will usually lead to you look like a potential sleazeball when people inevitably check a few details of the CV.
(edit) added keynote to the list of prestigious designations that a program may designate a particular speaker.