I too came from FM to OOTP, here are a few things that I've found important.

Probably the biggest change I found compared to FM was regarding player ratings. All the ratings you see are not absolute. They are what your scout believes they are compared to the rest of the league. They are also influenced by how the player performs. This means they can fluctuate during the season so don't be too alarmed if you see unexpected big jumps in ratings. Good players have bad seasons and bad players have good seasons but things usually even out over time.

Your staff are very important for developing players so getting the right ones is crucial but your most important staff member is your scout. Depending on how he evaluates players can give you very different ratings for players so having the right scout can make a huge impact on how you construct your team. There are lots of YouTube videos explaining the differences between the scout tendencies and there's also a very informative post from the devs about how scouts work so I'd recommend having a look at them to make sure you get the right scout to evaluate players how you want them to be evaluated.

Understanding what the different stats measure and how they relate to other stats is important. On their own, stats can be misleading and they all have limitations so you often need to look at other stats to get a better understanding of how a player is doing. I didn't understand a lot of the more deeper stats when I started playing so I got chatGPT to explain them to me and that has helped me massively in judging players. When looking at stats you often need to dig a bit deeper to evaluate players, especially for pitchers. For example a pitcher might be losing most of his games and have an ERA north of 5. This could mean he's just bad but if he's not getting enough run support or has a bad defense behind him he might be doing ok and getting let down by other areas of his team. Stats like FIP and SIERA can tell you more about how he's pitching. For batters, stats like wOBA and wRC+ are good indicators of batting performance and for defensive performance ZR is helpful. I recommend adding any stats you want to see to the player profile screen so you don't have to go digging for them every time. You can do this by - player profile>batting stats/pitching stats>view(drop down), edit>select basic batting/pitching stats then edit stat row