Oral posture refers to a dialect or accent's "home base" in terms of articulatory settings - specifically the jaw, the lips, the tongue, the cheeks, and the velum (soft palate).
JAW: Slightly open and not super active in movement
LIPS: Fairly relaxed with a tendency to slight round the lips on vowels such as in the word THOUGHT and STRUT.
TONGUE: The tongue is fairly active, arching towards the soft palate for /r/ sounds, moving from front to back vowels on diphthongs, and the blade/tip of the tongue is active for intial, medial, and final /t/ sounds
In this video, notice the fairly neutral position of the lips, until the speaker rounds the lips just slightly for the word TALK.
Oral Posture Quick Tip: The Hesitation Sound
A clue to a speaker's oral posture can be their thinking or hesitation sounds, something every speaker has.
[ ɛ̽ ]
The audio clip provides different examples of the Dublin hesitation sound. For the most part, it is realized as a front, mid open, mid centralized vowel. Think about keeping the jaw fairly closed and the lips relaxed.