My father is 70 year old and retired. I am doing his tax return and he somehow owes the provincial (QC) 720$. His only income is pension + OAS for roughly 17 000$ Why?

CPP and OAS income is taxable. So are any amounts he might take from RRSPs.

Weirdly last year he was not taxed on it.

He was paid out a 3800 amount which then the government claimed back (due to error) which makes it to zero.

I am basically just filling OAS and T4A P (pension) for him in the province of Quebec.

I somehow remember previous year we have some deduction for his provincial with medication, is that still applicable? 

Is there any way to reduce his 720 amount owed?

Yes, there is, in the Provincial section of the return. Here is a couple of other things if they qualify for - Schedule B - tax relief measure and schedule K - insurance plan https://www.revenuquebec.ca/en/online-services/forms-and-publications/current-details/tp-1-v/

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GRANT FOR SENIORS TO OFFSET A MUNICIPAL TAX INCREASE
https://www.revenuquebec.ca/en/citizens/tax-credits/grant-for-seniors-to-offset-a-municipal-tax-increase/

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