It's worth mentioning that exterior Oak can be problematic as it has a tendency to split and crack fairly quickly once exposed to the elements. Another point worth mentioning is that oils tend to weather quicker on horizontal surfaces where moisture or water can sit for prolonged periods. They can be used but depending on where the property is situated and the amount of weathering it receives, may need semi-regular maintenance to retain the integrity and effectiveness of the finish. This is less of an issue if the window sills have an angled surface so that water runs off rather than pooling on the surface. In terms of exterior joinery there are a couple of options you could consider. These are...
Both of these products are durable and contain UV filters to slow down the greying process caused by the suns UV rays. These penetrating wood oils are water / moisture repellent and help to keep the wood nourished and supple, therefore reducing the chances of the wood drying out and warping.
In terms of UV resistance, a coloured product always provides better UV protection than a clear product. This is because the colour pigments within the product essentially help to block some of the UV rays from reaching the wood.
A product that can be used to improve water resistance prior to applying the Osmo oil above is Osmo WR Base Coat (4001). This is a protective base-coat that can be used with most of the oils in the 'UV protection oil extra' range. This product however is not recommended for use as a basecoat under Osmo UV Protection Oil Natural (429).
An alternative approach to using a wood oil and possibly a better option for Oak would be to use a coating-based system from brands such as Sikkens or Sadolin. These brands produce coating systems where a base-coat or colour is applied then over-coated with a highly durable top-coat. The benefit of these products is that they are more resistant to standing water and will require less maintenance than oils on a horizontal surface. This said, the top-coat will degrade over time and will require maintenance at some point to retain the protective properties of the finish.
To discuss the various products and options in more detail, we recommend that you contact us and speak with one of our resident wood care experts.