"When" and "if" are two possible synonyms for "only when" because they both imply a conditional relationship between two events. "Upon," "after," and "once" are other synonyms for "only when" that emphasize the requisite nature of the second event. These words suggest that a certain amount of time or action must elapse before the second event can occur. "Provided that," "so long as," and "as long as" can also be synonymous with "only when," as all three phrases are conditions that must be met before an action can take place. Whether it's a matter of timing, action, or a combination of the two, there are many ways to express the idea of "only when" in English.