I say this not meaning to be critical, I'm only trying to understand
I'm guessing English isn't your native language, right? I ask as I'm not following your post/question, and want top phrase a response accordingly.
IF you are asking about Studio Mode in OBS (where there is a Preview Window and the 'live' {composited view that streamed and/or recorded} View), then yes, the Preview Window is for the OBS operator only. The Preview window isn't a delay, it is a separate 'work space' to adjust one's video composition without the stream/recording seeing those adjustments being made. The OBS operator then controls when and how the Preview Window is made 'live'
- There are some native to OBS basic transition effects, and with 3rd party plugins, many more sophisticated transition effects/filters.
Is this the OBS functionality you are asking about?