My husband and I are both visual artists, who like to experiment with different fonts and shapes. Is it okay to pick a letter we like and explore names that way? Right now, we fancy the letter O—it is a perfect shape, it's a primitive symbol representing unity and the circle of life, it can act as a number, and so forth. By adding Os, you can transform any amount into hundreds and thousands and millions.
O boy! Yes, it is perfectly fine to look for names according to a letter, or a set of letters, that you like. In fact, I often recommend going "by the letter" to couples who are having trouble getting started in the naming process. Often letters can serve as a springboard, and act as a window or door in which to view your choices. By restricting yourself to a particular letter (initially at least), you are free to explore all the possibilities within that limit.
There are not that many O names on the Social Security's top 1000 most popular lists. For the boys, there are only 11 names: Owen, Oliver, Oscar, Omar, Orlando, Orion, Omari, Osvaldo, Octavio, Oswaldo, and Odin. (Many boys' names end in O, such as Leo, Enzo, Bruno, Theo, Hugo, Rocco, Valentino, Leandro, Alfonso, Santino, Nico, and Maximo.)
For the girls, only Olivia and Olive appear on the list. I also found Odette, Odessa, Odele, Octavia, October, Opal, Orly, Ofira and Ophira, Oksana, Olympia, Ondine and Ottilie. For the boys, I found Orson, Otis, Otto, Oren, and Oz.