As Salamu Alaikum Wa rahmatullahi wa barakatuh
Is it permissible to apply oil on eyebrows and eyelashes to make them thick.
Can we apply hair regrowth oil on head and what are the ruilings on applying beard oil for thick beard

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is nothing wrong with using pure oils to thicken the hair of the head, beard, or eyebrows, or any other hair on the body, because there are no mentioned texts that stipulate that this is forbidden.

In principle, all things are permitted [unless proven that they are forbidden].

What you mentioned in the question is not included in the prohibited changing of Allah’s Creation.

Allah Knows best.