Opportunity cost can affect any decision including the shortest possible horizons.
For eg: even a 1 month FD v/s investment in 30 days debt MF needs a opportunity cost analysis.
With regards to accounting, opportunity cost involves no problem as it is not accounted for in the accounting records.
Continuing with the earlier example: let say FD has 4% interest and Debt MF yield is 4.1%. Now the opportunity cost for investing in Debt MF is 4%. But in the books no such entry is passed.