Oneirism is a term that is used to describe the state of being in a dreamlike or hallucinatory state. If you're searching for synonyms for this word, you can consider various terms such as delirium, hallucination, trance, reverie, and fantasy. Each of these terms represents a state of mind that allows a person to go beyond their normal state of awareness and experience a reality that is out of sync with the world around them.
Delirium is a state of confusion, hallucination is a perception of something that isn't actually present, trance is a state of deep absorption, reverie is a state of daydreaming, and fantasy is a state of imagination. These synonyms can be used interchangeably depending on the context and tone of the writing, but each provides an opportunity to express a unique aspect of the dreamlike state that is associated with oneirism.