Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You need to send an entire oak (おく) tree in the mail, but it's too big. Imagine yourself dragging one to the post office. They tell you that you can't send that here... Go to the oak sending facilities.
I sent a letter to Mr. Smith.
I'll send you the photos later on LINE.
It’s getting late. Let’s call it a day. I’ll walk you home.