finasteride will stop (most) of the conversion of test to DHT via 5-AR enzyme. I don't think ostarine will be causing a huge surge of test to warrant using fina on the cycle. If you want to jump on fina for long term hair maintenance that's a different story. It takes 3-6months to see improvement in hair, and will work well for maintaining and thickening up what you have. Will not regrow in already receded areas. Also fina is useless to DHT derivative compounds like Winstrol, Masteron etc. Doubt any hair benefit will be seen on osta either..
For on cycle hair protection I use ru-58841 (research chem topical anti androgen) or 5% topical Spironolactone cream (S5). These drugs occupy the androgen receptor within the scalp blocking the excess DHT in your system from reaching it. However, for mpb sufferers, when adding extra androgen hormones the hair is always some what at risk.