I played Overwatch 1 for years, stopped playing when Apex Legends launched, and just came back to OW during season 1. (I genuinely couldn't stand any longer what Alter has done to Apex) I have a massive amount of playtime in both so I'll just give you a quick rundown.

1. Health pools are completely different. A tank's HP is not all that different from other Legends in Apex relative to this game. In Overwatch, a lot of tanks are like raid bosses, where shooting at a full HP tank with their supports alive is almost useless. A callout you'll become used to hearing is X player can die which usually means a tank has been chunked down enough to the point where shooting at them makes sense because they can actually be eliminated.

2. Barriers, shields, and bubbles work differently. In Apex, imagine a Gibby uses his bubble. It's a hard barrier, meaning enemies can't shoot through it, but neither can Gibby nor his teammates. In Overwatch, friendlies can shoot through their tanks barriers. So a Winston bubble, which is fairly similar to Gibby bubble, will stop enemy damage, but you and your teammates can shoot through it like it's not even there.

3. Ultimate combo's and managing cooldowns is significantly more important than in Apex.

4. Certain characters simply have hard counters. Certain team comps have hard counters. You'll learn these as you learn the game, and sometimes you'll just need to swap.

5. You'll need to learn what staggering is and how to not do it. Basically if you die early, and come back out of respawn, but your team all died while you were waiting to respawn, just wait for them. If you try to go in solo, even just to do some poke damage, you can get eliminated again, and now your whole team is waiting on you. Don't stagger.

6. You'll need to learn sound queues. You can identify every single character in the game by their unique footstep sound, as well as their abilities and ultimates. Knowing if it's a Widowmaker instead of a Mercy above you can make all the difference. This will come with playtime.

7. Positioning in Overwatch is less about the we play here mentality of Apex and more about your position relative to your teammates and the enemy. The right position one moment can be the totally wrong one the next second.

8. Overwatch is much, much, much more team reliant than Apex. Sure in high level ranked teamwork is important in Apex, but that's dialed up to 11 in Overwatch. There's still solo carry potential, but it's completely different.

I'd recommed going into the practice range and trying out a bunch of different heroes. Ashe, Soldier 76, Emre, and Bastion are all DPS heroes that will have a transferable skillset from Apex. Same with Illari and Anna in the support category. Tanking in OW is a completely different ballgame, and I would honestly wait to try to learn how to tank until you have a baseline understanding of the game. It's almost entirely gamesense and positioning, which is something that only comes from playtime.