I am getting NOTICES again and again....

I am running Admin LATEST ON Osclass v4.1

Plugin omega is missing the index.php file /home/sc***z/public_html/post*****ne.com/oc-content/plugins/omega/index.php in /home/sc****z/public_html/postf******ne.com/oc-includes/osclass/classes/Plugins.php on line 122

[18-Oct-2020 02:01:52 Asia/Kolkata] PHP Notice:  Undefined index: fk_i_category_id in /home/sc****z/public_html/postfr*****ne.com/oc-includes/osclass/helpers/hDefines.php on line 507

[18-Oct-2020 02:01:52 Asia/Kolkata] PHP Notice:  Undefined index: pk_i_id in /home/sc***z/public_html/pos******ne.com/oc-includes/osclass/helpers/hDefines.php on line 517

[18-Oct-2020 02:01:52 Asia/Kolkata] PHP Notice:  Undefined index: s_city in /home/sc****z/public_html/pos*******ine.com/oc-includes/osclass/helpers/hDefines.php on line 518

[18-Oct-2020 02:01:52 Asia/Kolkata] PHP Notice:  Undefined index: s_title in /home/sc*****z/public_html/pos******ne.com/oc-includes/osclass/helpers/hDefines.php on line 524

[17-Oct-2020 20:22:22 Asia/Kolkata] PHP Notice:  Undefined variable: plugin in /home/sco***z/public_html/po*****ine.com/oc-content/plugins/osclasspoint_updater/functions.php on line 270

[17-Oct-2020 20:22:22 Asia/Kolkata] PHP Notice:  Undefined variable: plugin in /home/sco****z/public_html/post****ne.com/oc-content/plugins/osclasspoint_updater/functions.php on line 334

Omega is backoffice theme, not plugin.

Omega is backoffice theme, not plugin.

Yes you are right but this what i get from DeBug Log.. I have no idea why it is showing this.

Because you had it uploaded as plugin.
Other errors are maybe by some corrupted data on your site or ghost listings. It simply says that i.e. item's category ID is missing, but that is not possible, listing without category cannot exists. It would be good to know from what file/section error came.