ōyamafuda (competitive karuta); the six cards with six-mora kimariji
In competitive karuta (kyōgi karuta), the six cards whose kimariji (決まり字) are six mora long. These are the hardest to take quickly because the unique identifying part is long.
See also: 決まり字 (きまりじ)
Because the ōyamafuda have long kimariji, they require extra concentration in a match.
Compound of 大 (ō, 'big') + 山 (yama, 'mountain') + 札 (fuda, 'card'). The term is specific to competitive karuta and refers to the six cards that are considered 'big mountains' because of their long kimariji.