Can Catholics pray to Old Testament heroes like Abraham, Moses, David, and Elijah as one would pray to Christian saints like Peter or Francis?
Certainly. The Catechism of the Catholic Church tells us, "The patriarchs, prophets, and certain other Old Testament figures have been and always will be honored as saints in all the Church’s liturgical traditions" (CCC 61). Statues of such Old Testament figures as Moses, David, and Elijah can be found in some Catholic churches, a reminder to ask the intercessions of these saints.
Many Old Testament saints offer compelling examples of faith, perseverance, and heroic virtue and are as worthy of our devotion and prayers as the angels named in the Old Testament—Michael, Gabriel, and Raphael.
A prayer called the Litany to Old Testament Saints can be found online; it includes invocations of holy patriarchs, kings, women, prophets, and martyrs, both as individuals (e.g., Abraham, David, Sarah, Elijah, Abel) and as groups.
Of course, whether praying to Old Testament saints or Christian saints, we are always asking their intercession, not praying to them as we do to God.