Am I required to offer expiation for an oath after saying, "By God Almighty, so that you fear and do not return to doing this thing," and "If she is sick, I will work on her behalf," and then I worked on her behalf while she was sick, knowing that I suffer from obsessive-compulsive disorder?

The validity of an oath depends on the intention of the swearer. If, when you took the oath, you intended to obligate yourself to perform the deed only when you are in good health, and not during your illness, then you will not be considered in breach of your oath by having someone else perform it on your behalf. An exception is valid if it is connected to the oath. If there was no intention of making an exception, then the oath follows its underlying reason, and you are not required to offer expiation. An example of this is someone who swore not to buy meat because of the crowd, but then found it without a crowd and bought it—he would not be considered in breach of his oath. Similarly, if someone swore not to eat meat because it was harmful, he would not be considered in breach of his oath by eating lamb.