The meaning of in the opinion of the Income tax officer under section 145 of the Income Tax Act is not defined in the act itself. But we can take guidance from decisions by various high courts & supreme court. Here is an excerpt of the court’s order in which the Hon’ble Supreme Court tried to explain the meaning of in the opinion of the Income-tax officer.
Meaning of in the opinion of the Income tax officer defined by court
The phrase “in the opinion of the Income-tax officer” used in section 145 of the Income Tax Act came up before the Hon’ble Supreme Court in CIT Vs. Mcmillan & Co.  33 ITR 182 (SC) and it explained
Meaning: The words ‘in the opinion of the income tax officer’ in the proviso to section 13 of the 1922 act, do not confer a mere discretionary power, but in their context impose a statutory duty on the ito to examine in every case the method of accounting employed by the assessee and (i) to see whether or not it is regularly employed, and (ii) to determine whether the income, profits and gains of the assessee can properly be deduced therefrom.
Section 145 of Income Tax Act
Method of accounting.
145. (1) Income chargeable under the head “Profits and gains of business or profession” or “Income from other sources” shall, subject to the provisions of sub-section (2), be computed in accordance with either cash or mercantile system of accounting regularly employed by the assessee.
(2) The Central Government may notify in the Official Gazette from time to time income computation and disclosure standards to be followed by any class of assessees or in respect of any class of income.
(3) Where the Assessing Officer is not satisfied about the correctness or completeness of the accounts of the assessee, or where the method of accounting provided in sub-section (1) has not been regularly followed by the assessee, or income has not been computed in accordance with the standards notified under sub-section (2), the Assessing Officer may make an assessment in the manner provided in section 144.
In this article, you can get guidance from the supreme court on the meaning of in the opinion of the Income tax officer under section 145 of the Income Tax Act.
Updated up to Finance Act 2021.