the 'o' changes to 'ue' if the 'o' occurs in a stressed syllable

discordar - to clash (conflict of colors)
acordar (hacer algo) - to agree (to do something)
acostarse - to go to bed [tener relaciones sexuales]
aprobar - to approve
colar - to strain, to filter
concordar - to agree, to reconcile
comprobar - to check, to prove
contar - to count, to tell
costar - to cost
denostar - to insult
desaprobar - to disapprove of, to reject
descontar - to deduct
discordar - to clash
encontrar - to find, to encounter
engrosar - to swell
mostrar - to show
poblar - to settle, to colonize
recordar - to remember
reencontrar - to find again
tronar - to thundar