My main partner prides herself on her "tiny rosebud of a vagina"...whereas I am wishing she would open up more for some generous pumping. We use oils, and I spend lots of time with tongue and gently fingering...her position of comfort, from behind, soon brings her to orgasm and I feel her opening up wonderfully BUT she has had enough pushing by then. Help us understand what to do to help her open more fully earlier.
It sounds like the actual point right before or during orgasm is when you feel she is the most open and relaxed. This is the plateau phase, where blood flows to the genitals and the vagina continues to expand. If this is the case, perhaps you may want to try bringing her into the plateau phase and/or orgasm before diving into full-on penetration. You can do this via clitoral stimulation with fingers, your tongue, or a vibrator. If she prefers internal stimulation (i.e., the g-spot) in order to reach orgasm, try using a long, slender toy such as the Crystal Wand or the vibrating Oasis Slim. The vibrations can increase blood flow and circulation, which may also help to relax her body further before moving on to full penetration. Once you get into some heavy thrusting, if she feels the discomfort again, take a break from penetrative sex and try bringing her into the plateau phase/orgasm once more (i.e. multiple orgasms!).
Also, while it sounds like you are on the right track with plenty of lubrication and foreplay, you may also want to spend more time relaxing her vaginal muscles. You can do this with fingers by starting small and working your way up. Start with one lubed finger in the vagina, and when she is ready try inserting two or three fingers while gently moving them in whatever way she prefers. The Oasis line of vibrators also provides a 3-vibe system that allows you to start small and work your way up to the largest wand.
Keep in mind that many women experience tightness and/or discomfort during penetrative sex, especially when heavy thrusting is involved. Age (see my post on Toxic Sex Toys and Vaginal Discomfort for more info), size, and personal preference may play a role in her experience with penetrative sex. Be sure to communicate with your partner in order to find the right positioning and form of play that works for both of your bodies.