Is it ok to completely rebrand a software under osl 3.0 (change its logo, footer etc.) and sell it as SAAS, providing the original source code and modified source code both on the GitHub to public?
The OSL 3.0 license is an open-source, copyleft, copyright license.
This means that the license gives you the right to make any change you like and to distribute the result or to use it in any way you see fit.
As it is a copyleft license, when you distribute copies of the modified software, you must do so under the same (OSL 3.0) license and you must give the recipients of the software access to the source code. If you use it in a SaaS offering, this is only relevant for the portions that get executed on the user's computer, as only those portions can be said to have been distributed.
In fact, it may be a good idea to completely rebrand the original, because the OSL license does not give you trademark rights to the branding of the original software. Without rebranding you would have to negotiate those trademark rights separately. Just make sure you keep the license and attribution information in the source code intact, as that is required by the license.