Taxation of a OPC company after 01/10/2019

Section 115BAA of Income Tax Act,1961 Give the benefit of a reduced corporate tax rate for the domestic companies. ... The Effective Tax rate being 25.168% from the FY 2019-20 (AY 2020-21) onwards if such domestic companies adhere to certain conditions specified.

Any Domestic company including OPC can claim benifit of Section 115BAA.
A person who incorporates an OPC after 01/04/2020 can also claim tax rates u/s115BAA.

Sec 115BAA benefit for lower Tax rate is given to all domestic Company ,

Domestic Co. includes OPC Co. also
as definition is prescribed below

As per Section 2(22A)​, "domestic company" means an Indian company, or any other company which, in respect of its income liable to tax under this Act, has made the prescribed arrangements for the declaration and payment, within India, of the dividends (including dividends on preference shares) payable out of such income

Soo you can claim the benefit by filling Form 10IC upto due date of return filling