The Igrot Moshe (Vol. 4, 99) was lenient regarding the law of a “guardian” (a reminder) for the counting of the Omer. He argued that it is impossible that a guardian or a fixed Minyan would be effective for the Shema and prayer (as ruled in Siman 232) but would not be effective for the counting of the Omer. Furthermore, as the Ga’avad Rabbi Fisher wrote in Shu”t Even Yisrael, an alarm clock is also considered a “guardian” because it serves as a reminder.