Regarding Frederick F. Allen and O.P. Allen, I am a direct descendant of Frederick's and O.P. was my grandfather's brother (that making him my great-uncle). Frederick Freeman Allen was my great-great-great grandfather.
I grew up with O.P. in my life and since I was the only one who expressed an interest in the family genealogy, he more or less "passed the torch" on to me. I am fortunate that he was a Mormon who kept massive records of our family tree!
I have quite a lot of information on the Allen family that are Frederick's descendants, including many photographs of people and grave stones, including Frederick's.
I am particularly interested in getting more information on Frederick's ancestors that came from Ireland. I know that Frederick's father and mother were John Allen and Mary Davis, and John and Frederick came from Ireland and show up on the the Beufort County, NC 1820 census, along with several other Allens (listed on the same page) including Reuben Allen, who would have been Frederick Allen's brother. Frederick Allen moved to So. Illinois, having bought several hundred acres there through the "Bit Act" and shows up on at least the 1840 census in So. Illinois (I can't remember at the moment if he is on any earlier census there). Reuben stayed behind in South Carolina and signed as a witness to John Allen's will.
If you are interested in more information or if you have any further information, feel free to e-mail me!