We arrived at the parking lot of Onion Mountain Park at around 2 pm and proceeded to get our bags packed for the hike. I saw in the other review, that it is classified as a walk-in but it's definitely more of a hike. We ended up finding a very primitive spot off the blue trail (on top of the mountain), where we settled down for the night. The spot is not very big so make sure to bring a smaller tent (no group sites) but has a firepit already set up. We set up the tent and started a fire (very easy as there are sticks and wood everywhere). Overall a very good camping trip, and we highly recommend it.
P.S - There are a lot of bugs so be prepared, and we did put a bear bag about 100 feet away.