* In informal writing and conversation: It's perfectly fine to start a sentence with "Oh." It's often used to express surprise, understanding, or to introduce a new thought.
* "Oh, I didn't know you were coming."
* "Oh, that makes sense now."
* In formal writing: It's generally considered less formal to start a sentence with "Oh." While it's not grammatically incorrect, it might sound a little casual in a formal setting like an essay or a business letter.
* Instead of "Oh, the weather is beautiful today," you might say, "The weather is beautiful today."
In both cases, it's important to use "Oh" appropriately. Don't overuse it, and make sure it fits the tone of your writing or conversation.