This is what Jean Sutton and Pauline Scott say about OA and OP positions in their book Optimal Foetal Positioning:
The OL/A positions are the most favorable for a fetus to settle in prior to labor commencing. Of these positions, the LOL/A is the most common. The reason for this is that the maternal uterus lies with its fundus tilted forwards and to the right side of a mother's abdomen. The fetal back leans toward the concavity of the mother's abdominal wall. Because of the weight from the fetal back, the fetal head is able to flex and align itself correctly so that the vertex enters the pelvic brim in the lateral position (LOL), rotating to the anterior (LOA) once in the brim. Occasionally, a fetus will lie in the ROL/A position. If the fetus is more lateral than anterior, the chances of it rotating to the occipito-posterior are very high.
and:
If a fetus is diagnosed as lying in the posterior part of the maternal pelvis (OP) prior to labor or just as labor commences, the labor process can verge on the abnormal. This is particularly so if the fetus is lying on the right side of the woman's uterus (ROL/P). In this position the fetal back lies toward the lumbar curve of the maternal spine. This, combined with the weight of the fetal back, means that the fetus has to strengthen its neck and adapt a more erect attitude (the 'military' position'). Because the head is de-flexed, the vertex presents at the pelvic brim in a larger diameter than the 'anterior' fetus. {there is an illustration in the book} If it cannot enter the pelvic brim, or if it fails to rotate into the OA position once it does manage to enter, the chances of an obstetrically managed labor and birth are very high.
Here is what they say about posture:
If a woman (primigravida) regularly uses upright and forward leaning postures, particularly during the last 6 weeks of her pregnancy (the last 2-3 for a multigravida), her baby is given an excellent chance of positioning itself into the OA position. This is because when the pelvis tilts forwards, it allows more space for the broad biparietal diameter of the fetal head to enter the pelvic brim. Most of these postures, especially those that are forward leaning, are positions where a woman's knees are lower than her hips...
And on positions to avoid nearing full term pg:
Relaxing in semi-reclining positions: If a woman sits with her knees higher than the hips, which happens when she slouches back in a sofa or armchair to rest, the angle of her pelvic brim to her spine is reduce from 120^ to approximately 90^. If the woman regularly uses these reclining postures during the crucial period when her baby is deciding to enter the pelvic brim ready for labor, it is almost inevitable that, if it is able to enter the pelvis at all, it will do so in the posterior part of the pelvis and consequently present as an OP.
They go on to say that bucket seats create the same situation as above, and that women should avoid sitting with legs crossed because it reduces the amount of space in the anterior part of the pelvis.
This book is really quite good, and they give many "midwives' tips" on how to rectify "mal-positions" during labor.
What does that mean? or are they just laughing at me in my notes 
