I finally had what I consider to be a pretty definitive conversation with a front desk manager (I think) about OLCI and upgrades. Whether it's specific to the brand and property, I don't know. (This was a Homewood in Chicago.)
Last week, 1 day prior to a stay, I was viewing room choices in OLCI. I phoned the hotel to ask a specific question about rooms - not about upgrades, just about which directions certain rooms faced. I gave my name, and the person said "Oh, you're Diamond...you shouldn't be in OLCI yet!" I asked why, and he said "If you OLCI, we cannot change your room. You cannot be upgraded. However, if you don't OLCI, our morning process will almost certainly move you to a Premium Suite. Then, after that happens, you should OLCI."
Now I'm going to assume there's an override in there - *somebody* at the hotel *could* override OLCI if they had to, but as a matter of practice they don't go through doing it just to give modest upgrades to Golds and Diamonds.
So that evening, I continued to watch OLCI. Rooms were available on floors 19 through 7. (Those were the top and bottom floors of this hotel.) One by one, they're disappearing, the 19's, then the 18's, etc., all the way down to the a single room on floor 7. I'm starting to worry: what if the premium rooms get sold out? Will I *not* get an upgrade?
He said to wait until 10AM the morning of the stay. Sure enough, when I logged in then, my reservation had been upgraded and I was able to select a Premium Suite on floor 19.
It's the first time a hotel has ironclad confirmed to me what I'd long suspected: OLCI kills your upgrade, at least in "normal" scenarios. Finding someone who can override it, or asking for an upgrade at the front desk when you physically arrive...those are certainly possibilities. But it sounds like your greatest chance of an upgrade is to hold out until the morning of the stay.
On my solo business trips, I still OLCI as long as I see a high floor / away from elevator room. I don't want to gamble, and I don't need the upgrade. But if you *want* the upgrade...you're better off waiting.