They drafted two OT last year. They didn't know one would have blood clots and the other would be claimed off waivers. I don't think it's fair to say it's bad planning. It just didn't work out as hoped.
That said, I'll copy my breakdown of Kolton Miller from the other thread about 2018 draft prospects, where the OT discussion is well under way:
You all mentioned Kolton Miller recently. I had written him off earlier in the season, because I disliked how passive he was in his set. He rarely punches, instead opting to catch rushers as they come, and his hands get too far outside, causing him to grab and occasionally hold. Well, I just finished a deep dive on his 2017 season, and I am incredibly impressed. Other than this technical flaw, which can be cleaned up (paging Scar!), his game is absolutely perfect for the Patriots.
Let's get one thing out of the way: his height isn't going to be a detriment. He didn't struggle with leverage in any of the games I watched. He has very good knee bend, and his core strength is leagues better than McDermott's from last year, the other tall and athletic UCLA tackle. We drafted him in the 6th, and he was one of my most sure-fire locks to the Patriots, based on his tape and athletic profile; he didn't make it in camp, but he's with Buffalo now. I always thought he lacked a little functional power, but that it would come with time. Miller is far better at absorbing contact and re-anchoring.
Miller is also quite solid in the run game, and UCLA often ran left in short yardage to take advantage of his ability to fire off the ball. Three things stood out positively to me: 1) his enormous range in space when pulling, and the ability to lock onto a target and make the block; 2) quickness to the outside -- he can beat the DE to the edge, and then turn and wall off the defender on outside running plays, which is tough for any tackle; and 3) whenever he finishes a block, he looks for more work to do. I hate passive line play, so this kind of attitude is always encouraging to me.
Against the pass, his kick step is low and wide. He just eats up ground in pass pro, easily shutting off the corner against speed. His redirection against inside counters -- one of Solder's few weaknesses at times -- is also shockingly good for a tall guy. His knee bend and relatively low base for his size allow him to mitigate the lack of leverage that his height might suggest. While he's not overpowering against bull rushes, he does display the ability to reset his feet, and he's got a better than advertised anchor. There's a lot of natural athleticism to work with, and if he can improve his punch and get his hands inside the blocker's frame, he can dominate against speed and power.
Overall, I see Miller strongly in play for 43. Much better player than I originally thought.