Is it OP? Maybe. I don't know.
But from quick testing and on paper it's really not as gamebreaking as people make it out to be.
That's at lvl 3. The ult starts with a cd of 80s at lvl 1 and then goes down quickly. Not only that, but there is also an extra cd specific to the champion he stole the ult from. He can't steal the same ult for 200% of its cd modified by Sylas' cd reduction (naturally, this is dependant on how much the enemy lvled their ult, even if Sylas has a lvl 2 ult he'd still get the champion specific cd of a lvl 1 ult if that champ only lvled their ult to lvl 1). If he steals an ult with a cd of 100s and he has 40% cd reduction, that means he can't steal from the same champ for 120s. Is that champ the only one around or the only one where it makes sense to steal from? Too bad, you have no ult during that time.
Edit: I forgot to mention that the minimum cd for stealing an ult is capped at 20s. This lowers his chances of being able to make use of that 9s ult (with cd reduction) because he will have to rotate between champs for that.
“What about counterplay?“
In a way, Sylas' ult is one of the most predictable in the game. That's because he has to steal it first and the range of that isn't that big either. The thing is that the stealing part is as obvious as it can get because he shoots his chains out and only after his chains have come back to him will he be able to use the ult. That's quite a bit of disruption on it.
Edit: King Leo mentioned that the stealing part of Sylas' ult might follow the same rules as other ults. While I didn't test it with Yasuo's wall, I can confirm that Sivir's shield blocks the stealing.
“But he can keep it indefinitely“
If he stole your ult, you will know it. So nothing is keeping you from saying so. Then nobody will be surprised by it. I have forgotten to properly check if there are any visual indicators for what ult he stole, but knowing Riot, there probably is one.
Edit: he has a little icon with the stolen ult next to his name. Thanks Albus!
Edit: It also appears that he can keep an ult for about 120s and then it resets again if he doesn't use it. I checked this with both Lux' ult and the "ult" of an enemy dummy ( i feared I would open a rift to another dimension after I did this, but the stolen "ult" which was just a black icon disappeared after 120s)
“He can have anybody's ult“
That's true. Can't say much about this one, only that there are a lot of ults which are incredibly gimped on him because he can only trigger them once and he lacks the interaction that some ults have with their champion's passive.