Treatment includes:

Abstention from chewing areca nut (also known as betel nut) and tobacco
Minimizing consumption of spicy foods, including chiles
Maintaining proper oral hygiene
Supplementing the diet with foods rich in vitamins A, B complex, and C and iron
Forgoing hot fluids like tea, coffee
Forgoing alcohol
Employing a dental surgeon to round off sharp teeth and extract third molars
Interprofessional treatment approach Treatment also includes following:

The prescription of chewable pellets of hydrocortisone (Efcorlin); one pellet to be chewed every three to four hours for three to four weeks
0.5 ml intralesional injection Hyaluronidase 1500 IU mixed in 1 ml of Lignocaine into each buccal mucosa once a week for 4 weeks or more as per condition
0.5 ml intralesional injection of Hyaluronidase 1500 IU and 0.5 ml of injection Hydrocortisone acetate 25 mg/ml in each buccal mucosa once a week alternatively for 4 weeks or more as per condition
Submucosal injections of hydrocortisone 100 mg once or twice daily depending upon the severity of the disease for two to three weeks
Submucosal injections of human chorionic gonadotrophins (Placentrax) 2-3 ml per sitting twice or thrice in a week for three to four weeks
Surgical treatment is recommended in cases of progressive fibrosis when interincisor distance becomes less than 2 centimetres (0.79 in). (Multiple release incisions deep to mucosa, submucosa and fibrotic tissue and suturing the gap or dehiscence so created by mucosal graft obtained from tongue and Z-plasty. In this procedure multiple deep z-shaped incisions are made into fibrotic tissue and then sutured in a straighter fashion.)
Other causes include:

Immunological diseases
Extreme climatic conditions
Prolonged deficiency to iron and vitamins in the diet

Pathogenesis
"Exposure to areca nut (Areca catechu) containing products with or without tobacco (ANCP/T) is currently believed to lead to OSF in individuals with genetic immunologic or nutritional predisposition to the disease. "This hypersensitivity reaction results in a juxta-epithelial inflammation that leads to increased fibroblastic activity and decreased breakdown of fibers. The fibroblasts are phenotypically modified, and the fibers they form are more stable, produce thicker bundles that progressively become less elastic. once the original loosely arranged fibrous tissue is replaced by the ongoing fibrosis, the movability of the oral tissues is reduced, there is loss of flexibility and reduced opening of the mouth. These collagen fibers are non degradable and the phagocytic activity is minimized. According to a  recent cross sectional study the time taken for return  of salivary pH to baseline levels after chewing areca nut containing mixtures is significantly longer in habitual users with  OSF when compared to unaffected users. Prolonged Alkaline pH induces death fetal fibroblast type and replacement by a profibrotic fibroblast. The patterns of intraoral fibrotic bands produced by alkaline chemical injury mimic those produced by areca nut chewing. Sharma et al., have equated the pathogenesis of OSF to an over-healing  wound, to explain its evolution as well as malignant transformation. Increased mechanical stiffness through YAP/TAZ pathway accelerates the malignant transformation of OSF.