"At the first opportunity" is a phrase that is typically used to describe doing something as soon as it is possible to do so. There are several synonyms that can be used in place of this phrase, including "immediately," "as soon as possible," "without delay," or "at the earliest moment." Other options include "promptly," "at once," "instantaneously," or "straightaway." Depending on the context, other similar phrases could include "right away," "right off the bat," "on the spot," or "straight out of the gate." Regardless of the wording used, the meaning remains the same - to take action quickly and without hesitation.