Odo of Rheims
For Brewer (p. 41), the existence of Odo's Letter corroborates the supposition that the events described by the anonymous de Adventu were, indeed, based on a historical occurrence.
"He claims to have been present when the mysterious John arrived in Rome and gave his narrations to Pope Calixtus II, as described in [De Adventu]. The De Adventu and this letter narrate the same event, but seem to be textually unrelated, which suggests that John's visit to Rome was a real occurrence, though little more can be said without dangerous excursions into speculation."