From: Rabbi Akiva Eiger
To: Unknown recipient
R' Binyamin Wolf Eiger rules that writing the Omer count does not fulfill the mitzvah of Sefiras HaOmer, and one who only wrote must recount verbally with a berachah. He proves this from the law of oaths (where writing is not equivalent to speech) and from the fact that a berachah was instituted on Sefiras HaOmer, which would be inappropriate if mere thought or writing could fulfill it. The luminaries present at the wedding feast concurred with this ruling.
Source: Shu"t Rabbi Akiva Eiger §29