‘Oral language’ is a predictor of reading ability and includes a range of skills. It refers to expressive skills, such as the ability to comprehend vocabulary, along with expressive abilities in putting words together to form grammatically appropriate phrases and sentences, and combining words together in meaningful ways.
https://eclkc.ohs.acf.hhs.gov/sites/default/files/pdf/big5-big-picture-oral-language-vocabulary-eng.pdf
What is expression?
Expression refers to a persons ability to make themselves understood in writing or while speaking. Many resources for developing writing can be used as verbal expression resources by simply having a student speak instead of writing.
Variations in oral language abilities account for differences in reading comprehension more significant than age, nonverbal ability or non-word reading.
https://www.researchgate.net/publication/41387471_Vocabulary_Is_Important_for_Some_but_Not_All_Reading_Skills
Why does expression matter?
Students who struggle with verbal expression may be labelled as shy or hesitant to participate. They may struggle to organize their thoughts or have a hard time find the words they want to use.
Educators cannot assume that all students enter school with adequate listening and speaking skills. Although teachers employ all four language modalities in instruction, for example, through oral discussions (oral expression), lectures (listening comprehension), writing assignments (written expression), and reading assignments (reading comprehension), they are less likely to be aware of the importance of taking individual differences among learners in these four language skills into account in planning instruction and assessing response to instruction.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3068836/
How can I tell if expression is an issue?
Issues with verbal expression can sometimes appear to be reading comprehension issues. Parents will frequently have the sense that their child understands what they are reading but just can’t figure out how to explain what they read. One way this shows up is a student who can immediately recognize the answer to a multiple choice answer (without using test taking strategies) but cannot generate the answer in an open question format.
Screener: https://www.neuhaus.org/reading-challenges/screening-measures