Many issues involving digestion have been eased or resolved by using the essential oil blends (Digestion Support and Tummy Soothe) or Peppermint linked above. Click on each to read through testimonies.

Single Oils that may soothe digestion issues:
Basil ct linalool (Ocimum basilicum)
Cardamom (Elettaria cardamomum)
Coriander Seed (Coriandrum sativum)
Eucalyptus radiata
Ginger (Zingiber officinale)
Lemon (Citrus limon)
Orange (Citrus sinensis)
Peppermint (Mentha piperita)
Star Anise (Illicium verum)

Reference
Worwood, Valerie Ann, The Complete Book of Essential Oils and Aromatherapy, New World Library, 2016, page 339.