I have column O which is date format 00/00/0000, column Q which has
either the word 'Date' or a numerical figure to 1 decimal place and
column I which has a numerical figure to 1 decimal place. I need
formula in column S that looks at Q and if it shows date then it needs
to return N/A in the S cell and if it has the numerical figure to 1
decimal place it needs to do the sum Q-I and give the result in the
cell. What should my formula be?

If the figures to 1 decimal place are already rounded, maybe this would work:
=IF(ISNUMBER(Q1), Q1-I1, "N/A")
Enter it it S1 and copy downward. If Q1 and I1 aren't rounded yet, apply the appropriate rounding function to them inside the formula.

Apparently, column O plays no role in the calculation.