Hi YW fan:
So the answer may be related to the Machlokes Rishonim and Poskim whether there is a single Mitzvah to count the Omer (all 49 days) or whether there are 49 individual (sub-)Mitzvahs; i.e., to count each of the 49 days. Such a discussion will lead to whether we are counting each night as part of the greater whole (then it might be Ba’Omer — in the Omer) which means in the entire Mitzvah of the 49 days. Or perhaps, like the 2nd opinion, each night is independant of itself and therefore, maybe La’Omer — to the Omer, to the Mitzvah count itself. Some of my humble thoughts on the issue.
Tzarich Iyun — Warrants further study.