Sorry if it is not even allowed to discuss about this topic here but I really want to get clarification with it.Cause some say it is allowed and some say no. I thought this is the best place to get clarification.
Is that allowed to use some game assets of oblivion in skyrim for example textures?

100% not allowed mate

Both the long and short answer are "No, it's not allowed".
Not sure why though.

It's not allowed, period. Uploading them to mod sites will get you banned, and possibly in trouble with Bethesda.

The reason being is that the resources from one game are licensed for that game only. People made them that might no longer be working with the company, some might use licenced third party assets just to name two examples.

How come we are even allowed to upload modified textures and models at all, really? if you think about it. Even if they are intended for the game they originally came from, anyone can download them. Without even owning the game..

But without owning the game there's not much they can do with them.

Modifying textures and models is ok - as long as they are from the game you are uploading them for.
Using assets from other games isn't allowed because they are other games.

It is ok if game developers give their approval, for example you can see tons of the Witcher models and music ported to Skyrim becuase devs game their permission

I'm sure they set aside loads of time just for this purpose...

Not up for discussion on this forum.