There is not a specific grammar tool in Word Online, but there are a few ways to check grammar. One way is to click the Review tab, then click the Spelling & Grammar button. This will check the spelling and grammar of the entire document and highlight any errors.
Another way to check grammar is to highlight the text you want to check, then click the Review tab. Under the Proofing section, click the desired grammar checker, such as Grammar or Contextual Spelling. This will check the grammar of the highlighted text and highlight any errors.