ω, B, W symbolize Christ as the end of everything being the last letter in the greek alphabet. "I am the alpha and the omega" says the Apocalypse.
The omega representation had a difficulty by to be a greek letter that had no equivalent in latin. It is represented in various ways besides the normal omega: e uncial horizontally, B horizontally, M latin letter inverted, W.
Santa María de la Horta (Zamora), Puilampa (Zaragoza), San Miguel de Almazán (Soria), San Zoilo (Carrión de los Condes, Palencia),San Juan del Mercado (Benavente, Zamora), San Vicentejo del Condado de Triviño (Burgos), San Juan Bautista de Castiliscar (Zaragoza).
Read more in The stonemason’s signs in the Romanesque churches in Spain.