There is a famous dispute over the proper wording while counting the omer.
Some have the custom of saying "yamim LA'omer," while others say "yamim BA'omer."
What's behind the two customs?
The Shulchan Aruch (489:1) doesn't mention La'omer or Ba'omer when discussing the halachos of counting the omer.
The Rema adds the word Ba'omer. However, later, when discussing the laws of sefira, the Shulchan Aruch (493:1) uses the word La'omer.
The Taz (489:3) says the custom in his town was to say Ba'omer, and it's better than la'omer. He argues la'omer means "from the time of the offering of the korban omer." We know the korban omer was brought on the second day of Pesach, so how could the proper wording be la'omer if, on the first night of counting, we still need to bring the korban ome? Ba'omer means "during the days of the omer," meaning it's not counting from the actual korban, just the days of the omer, so even the first night, the wording is correct.
Chok Yaakov (489:9) argues on the Taz and brings many Rishonim (including Ran, Rashba, Kol Bo, and Tanya) that say La'omer.
He then explains that Ba'omer is incorrect because it implies that the Korban is being brought now, but it's only brought on the first day. La'omer means "from the DAY we bring the omer," and even though we didn't bring it on the first night, it's still the day of the omer.
In practice:
Gra (M.R. 69) rules Ba'omer.
Ba'er Hetev (489:8) brings the Arizal and Shelah that hold La'omer.
Mishnah Berura (489:8) says most poskim hold La'omer.
Chok Yaakov (489:9) says a person should not change his custom, as each has merit.
It's worth noting that the significance of this discussion is not so pressing because even if one leaves out the word entirely, the counting is still valid.
(Bais Yosef, M.B- 489:8).