In-house maintenance staff can typically handle the two periodic tests needed for dry-type transformers: 1) insulation resistance testing, and 2) core ground (if applicable). One advantage of oil-filled transformers over dry type is you can tell a whole lot more about their condition. However, the tests you must perform to get this information are typically beyond the capabilities of in-house maintenance staff.
Use a qualified testing firm to conduct a full program that includes the dry-type tests plus:
Required frequency ranges from three months to five years, depending upon transformer condition and criticality. When scheduling the shutdown, allow for contingencies.