If I take Prestarium and Nebivor and supplement Ashwagandha and Bacopa monnieri, can I start using oregano oil?

Yes, you can. There are no contraindications to the simultaneous use of oregano oil and the hypertension drugs and dietary supplements mentioned in the question. But is there really a need to use so many dietary supplements at the same time? I suggest that you consult a family doctor or dietician whether the use of all these preparations is justified. If you do not have the opportunity to take advantage of the advice, I enclose a link to online teleadvice, e.g. via the Gdziepolek.pl portal:
https://www.gdziepolek.pl/telekonsultacja/z-dietetykiem

I encourage you to inform your doctor about all medicines and dietary supplements used, which will allow you to choose a safe and effective pharmacotherapy. Only your doctor can assess whether the combination of these medicines is safe and appropriate for your current clinical condition.

The answer regarding ashwagandha and hypertension medications is in the following thread:

https://www.gdziepolek.pl/opinie-i-pytania/105960/witam-przyjmuje-lek-prestarium-oraz-nebivor-na-nadcisnienie-w-ma

And the answer about Bacopa monieri can be found here: https://www.gdziepolek.pl/opinie-i-pytania/107997/witam-serdecznie-czy-biorac-leki-na-nadcisnienie-takie-jak-prest