Below are some YouTube channels or other online tutorials that may help you to develop your skills further. 
Click on the image to go to their website.

Michael James Smith: Realistic landscapes

Mark Carder (Draw Mix Paint): realism in oils

Wilson Bickford: Paint landscapes

Kevin Hill: Oil and acrylic landscapes


Farbi Flora: Expressive painting techniques

Have you come across a really good online tutorial? Let us know about it here...
Thank you!