Charles Ellicott Commentary Exodus 16:36

"Now an omer is the tenth part of an ephah." — Exodus 16:36 (ASV)

Now an omer. —The "omer" and the "ephah" were both Egyptian measures. One—the latter—continued in use among the Hebrews, at least, until the captivity (Ezekiel 45; Ezekiel 46); the other—the omer—fell out of use very early. Hence this parenthetical verse, which is exegetical of the word "omer," and may have been added by the one who completed Deuteronomy, or by some later editor—perhaps Ezra.