In reality, it doesn't matter which way you do it as long as you keep the cues short and sweet.
Most of the time, T-Ok! is used in both situations simply because the students become trained to know what to do whenever you say that. You usually give a BRIEF, "Talk to your neighbor about _________ ... TEACH!" and they're off!
The one reason I might suggest sticking with T-Ok! in both situations is just to keep it simple... that way there's not aNOTHER cue and response for them to remember. think about it -- In a completely WBT-style classroom, we have Class! Yes!... Teach! Okay!.... Switch! Uh-Oh, SWITCH!... Papers! PapersPapersPapers! (Or page ___, or seats, or lines...) ... Hands and Eyes! Hands and Eyes.... Mirror! Mirror!... Mirror Words! Mirror Words!.... Mighty Cheer! OH YEAH!... Mighty Groan! AWWWWWW!....
Those all serve a clearly different purpose, and therefore are necessary, but it's already alot to remember, so you may find you are saving your class some confusion if you keep it simple... all pair-sharing, whether it's teaching your neighbor or discussing with your neighbor...cued by a simple "Teach!".
However, with that said.... there's no 'rule' about it. The only thing that matters is that you are chunking those lessons and constantly having the students teach each other or discuss with each other briefly and frequently. If you're doing that, you could say, "Cow! MOO!" and it would still work.