If I bolt a transistor to aluminum for an electrical and thermal connection, are there any oxidation concerns for the electrical contact?
Galvanic Corrosion is something that you should be aware of when mechanical aspects of electronics design come into play.
Find out what metals are involved in your assembly and select screws, nuts, washers and heatsink materials that are most suitable.
Also, it is occasionally difficult to make a good electrical contact with aluminium parts (heatsinks or enclosures) due to natural or anodized oxide layers. One way to break through is to use a "spring" or "locking" washer to gouge into the soft aluminium and penetrate the oxide layer to form a gas-tight seal. The materials involved are still a concern in safety critical applications.
Aggravating factors can be operation in a moist or salty environment or where harsh, prolonged vibration can cause fixings to become loose over time and weaken electrical connections.
Conformal coatings are another option available for PCB-level ingress protection. The PCB assembly company can add a sprayed-on layer of sealant that provides moisture and particulate shielding. These can be customised for your application. It is a common process and shouldn't be very expensive. However it may cause some issues when diagnosing faults in post-production testing.
EDIT: (The OP indicates that this is a vibration sensitive vehicular application)...
Vibration can be mitigated by mounting the entire assembly on rubber shock-absorbing washers where the outside of the enclosure meets the vehicle chassis.
Internally, individual components can be secured by other means such as using cable ties (aka "zip-ties" or other names) or silicone based flexible adhesives to absorb, dissipate or attenuate shocks to the system.
Depending on your application it may be appropriate to use a solid potting compound to encapsulate the entirety of your internal circuitry to prevent moisture ingress or shock from damaging your circuit. Engine ECU/EMU systems are often fully encapsulated in this manner as they are safety critical systems.