The crystal field splitting energy ($\Delta_o$) depends on the nature of the ligand. Ligands are arranged in a series called the spectrochemical series based on the increasing order of the field strength they produce.

According to the NCERT textbook, the order is: $I^- < Br^- < SCN^- < Cl^- < S^{2-} < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < edta^{4-} < NH_3 < en < CN^- < CO$

Comparing the specific ligands given in the options:

1. $SCN^-$ (thiocyanate) is a weak field ligand (appearing early in the series).
2. $F^-$ (fluoride) is a weak field ligand but stronger than $SCN^-$.
3. $C_2O_4^{2-}$ (oxalate) is stronger than $F^-$ (it produces a larger splitting).
4. $CN^-$ (cyanide) is a very strong field ligand (appearing near the end of the series).

Therefore, the correct increasing order is $SCN^- < F^- < C_2O_4^{2-} < CN^-$ .