Oldershaw, from Burlington, Ont., was second in the men's C-1, 1,000 metres, 1.721 seconds behind Brazil's Isaquias Queiroz dos Santos. Mexico's Jose Cristobal was third.
In the K1 1,000 metres, van Koeverden was 2.065 seconds behind winner Jorge Garcia of Cuba. Daniel Dal Bo of Argentina was second.
The four-time Olympic medallist from Oakville, Ont., led until the final 300 metres but faded to the finish line.
Canada normally sends a B team in canoe/kayak to the Pan Ams, so this marked the Pan Am debut for both van Koeverden and Oldershaw.
Van Koeverden, 33, won gold and bronze at the 2004 Olympics in Athens, plus silver in both the 2008 and 2012 Olympics.