There's an argument whether annulling an oath needs an excuse. Even if there needs to be an excuse, the excuse could be something weak like "I didn't know I shouldn't take oaths".
What's the point of an oath which could be cancelled at any time?
If the oath was made with the intention to nullify it, i would assume it is not an oath, as he never intended to keep it (and hence it was not le'esor issur) . So, we're talking about a case were the person meant to keep it.
In order to cancel it, it must be something new. Although the reason to remove an oath can be weak, the process itself is not a joke. For example, it cannot be nullified by just anybody.
Further, this is only an oath that a person makes on his own. Once there is another party involved, the other party may be required to allow nullification.