Official release 3.9.0
And I would not recommend you to use the Osclass Evolution branch

3.9.0 IS NOT an official release neither 3.8.1.
The LAST official release is 3.8.0 and thats it, they stoped in September 2019, after that came out 3.8.1. wuth removed market etc and later 3.9.0.

P.S. I dont know but I already instaled it, nothing has changed only the admin is different... I have the same problems I had with 3.8.0, tried to solve them with 3.8.1 and 3.9.0, nothing has changed.

https://github.com/mindstellar/Osclass
You can try develop branch of this repo.
I can confirm  it has a lot of new  features and bugs solved (I personally i'm involved  in and working with osclass script from 4 years ago )
https://github.com/mindstellar/Osclass/issues?q=is%3Aissue+is%3Aclosed

https://github.com/mindstellar/Osclass/issues/160

https://github.com/mindstellar/Osclass/issues/140

https://github.com/mindstellar/Osclass/issues/225 .
If any other version found please do invite "the developer" to make a pull request on mindstellar branch.
This way we can see developer quality,and his level of experience

I actually like the 4.1.0 version. Works good for now. I am not an dev so I cant do anything 🙂 Otherways i was probably going to do so long time ago and make my own version of osclass with all changes I wanted... but unfortunately I am just SEO idiot :d

First .. i would not call it 4.1.0
Its more than 3.8.18 🙂
Would be better if that evo osclass branch will go to github so we all contribute
That is not a one man job to keep up to date.
Not a good practice to stick his own products on that  bootstrap admin theme.
Download do not work ,  so called 4.2 zip has 627 kb only
Beside that not so many php changes , and still osclass has many  unsolved ( php , code logic ,features) issues .
Would take years for him (evo developer)alone to make it progress.
So for you going with evo would bring some buttons here and there ,some new features , (maybe good ones)  , but no high level of php develop.
Thats first
And second .. he cannot keep track and solve issues by himself (only him)
Osclass had an army of developers and still  was full of unsolved issues like i mentioned in earlier post

I advise you and all others  (including evo developer/s) do make some pull request, create issues on navjottomer mindstellar branch.
Including that admin theme as plugin .
So we can keep a nice updated version of osclass

Did some benchmarks .  As you can see evo  version is way behind .
http://localhost/osclassevo/index.php?page=item&action=item_add
 

[duration] => 0.17000008
            [memory] => 7.77 mb
            [peak] => 8.38 mb
       

http://localhost/navjotommer/Osclass/index.php?page=item&action=item_add

 [duration] => 0.1099999
            [memory] => 6.75 mb
            [peak] => 7.32 mb

http://localhost/osclassevo/index.php?page=search&sCategory=1

[duration] => 0.29999995
            [memory] => 7.79 mb
            [peak] => 8.32 mb

http://localhost/navjotommer/Osclass/index.php?page=search&sCategory=1

[duration] => 0.12999988
            [memory] => 6.27 mb
            [peak] => 6.83 mb

http://localhost/osclassevo/index.php?page=item&id=1

[duration] => 0.21000004
            [memory] => 7.84 mb
            [peak] => 8.34 mb

http://localhost/navjotommer/Osclass/index.php?page=item&id=1

[duration] => 0.1099999
            [memory] => 6.29 mb
            [peak] => 6.84 mb

What do you know and if you tested Osclass v4.0.0 from osclasspoint?

Same for osclasspoint and evo or any other 
cheap clone  intent  :
Do  join us on github and feel free to  make pull requests on comunity fork to keep same codebase, plugins and themes compatible .
Stop cloning osclass, adding 2 buttons and make it a new version .
Do solve real bugs.

I will echo the sentiment that it's confusing and disheartening to see so many splinters of the original OSClass and so little cooperation between developers.  First I got an email from Navjot announcing v5.0.0 beta.  Then I got one from OSClassPoint announcing 4.1.  Yet another is peddling a v4.3?  All are seemingly doing their own thing looking for brownie points.

Now I'm not so sure I want to try upgrading from mindstellar/Osclass 3.9, or even continue with OSClass at all considering the apparent divisions that seem to be emerging.

OSClass is at a crossroad. The question is can developers unite for the good of all?

I will echo the sentiment that it's confusing and disheartening to see so many splinters of the original OSClass and so little cooperation between developers.  First I got an email from Navjot announcing v5.0.0 beta.  Then I got one from OSClassPoint announcing 4.1.  Yet another is peddling a v4.3?  All are seemingly doing their own thing looking for brownie points.

Now I'm not so sure I want to try upgrading from mindstellar/Osclass 3.9, or even continue with OSClass at all considering the apparent divisions that seem to be emerging.

OSClass is at a crossroad. The question is can developers unite for the good of all?

Navjot's code is on Github. Everyone can contribute, everyone can see the code, everyone can report bugs.

Other sites are doing closed source whose code you can't see or contribute to. As I just said in another topic - Their main goal is to monetize Osclass and push people into their ecosystem and their markets & plugins.