The difference between Oil cooled Transformer and Dry type transformer is so much that we need to write a full article. A brief of points are appended below.
Advantages of dry type transformer
Low maintenance cost. Needs only to be cleaned of dust.1.
OCT needs regular oil checking, filtration and replacement of oil which is
very costly and need high recurring expense.
2. Can be installed very close to load center hence can save low voltage
Transmission losses, which is substantial recurring saving and advantage on energy conservation step. (Considering the present day energy shortage).
3. Need comparatively less space for installation and can be integrated with VCB control unit to make a single unit to save space for HT switching panel
4. Reduce fire hazard in substation since no oil is used.
Non-repairable unlike OCT if there is damage to winding if it is cast resign type.
The safety of over heating is through sensors which should be monitored regularly to ensure safety against overloading and fault conditions.
Regular dust cleaning if not carried out, can result in damage to transformer.
The cost difference and pay back period will depend on each case study which you will have to calculate depending on site conditions.