I’ve found out that if a verb starts with “ont-” it could be one of the two following cases:

1. Adds “away”, “un-” or “release” to the meaning of a verb
nemen – to take
ontnemen – to take away, to deprive

doen – to do
ontdoen – to undo, to discard

2. Indicates the beginning of action or changing of the state
branden – to burn, to be on fire
ontbranden – to ignite, to flame up

staan – to stand
ontstaan – to come into being, to arise, to originate, to start