Advice on Ozymandias

Personally I would say Merlin. Literally the reason buster crit teams are meta. He just objectively provides far more for a buster crit team than Ozy ever could. Not only that, he is good in virtually every team comp, so he's useful even if you completely overhaul your team. That's not to say that Ozy isn't a top class servant - he is, but gameplay wise there is a reason Merlin has become a meme for how OP he is. The only reason not to go for Merlin is if you prefer Ozy, or if you want to make the game more challenging for yourself.

Raikou and Ozy are both damage dealers. It would be better to focus on one damage dealer with a caster support like Waver or Hans. If you rolled Ozy, you can change Raikou with him during boss fights and use Raikou on farming purposes.

It is also advisable to roll for Merlin due to he is limited while Ozy can spook you anytime.

Merlin is broken and limited. In my opinion you should wait for Merlin but also you can go for Ozymandias and use friend with Merlin it only depends on you.