Examples of la- and ba- prefixes are common in chumash, e.g., yom XX bachodesh and yom XX lachodesh. As I've written before, puk chazi mai 'ama dvar: I will change to la-omer after lag la-omer or after tish'a l-Av or 'asara le-Tevet. Until then I stick to ba-omer because that's the way people talk the language. David