I've heard rosemary oil when applied to the scalp is as effective as minoxidil for regrowing hair. Is that true?

Yes, Rosemary oil is an antioxidant, stimulates circulation, and promotes hair growth.  I suspect some products work better in some individuals.  I'm not sold on the hair regrowth potential for minoxidil alone.  Very few people are allergic to minoxidil.  most are allergic to the vehicle the minoxidil is delivered in.  Oral Minoxidil, 1.25 mg per day is also a possibility.  You should probably look at a number of products to help you maintain your hair.  You can look for a shampoo that has many different products.  Look at WNT-act too.  It's a natural, plant based hair growth promoter.  I doubt any of these products alone will regrow your hair.  Your primary goal in treating your hair loss with tropicals is to retard the aging process in your hair follicles.  you want to keep your follicles youthful for as long as possible.  

There are some case reports that rosemary oil has some Effectiveness for androgenic Alopecia. It is most likely not as effective as minoxidil. May I recommend that you try the minoxidil foam as it is less irritating. Furthermore, your hair loss specialist can compound minoxidil in an oral capsule which minimizes scalp side effects. It is also possible to use a lower oral dose if you are concerned about other side effects.PRP hair md.com

There are no studies to suggest this. Off the shelf remedies like this have no science behind them, and are not subject to the testing FDA approved medications like minoxidil have to go through.  Best to stay with what has actually been tested, and minoxidil and finasteride are the only medications that have FDA approval for hair loss.

There has really been only one reasonably sized study in the medical literature comparing rosemary and minoxidil. Rosemary oil may have benefits in the treatment of male balding. A 2015 study by Panahi and colleagues found rosemary oil application to the scalp to be similar effectiveness to Minoxidil, at least for men. Whether the data are reproducible and whether they stand the ultimate test of a randomized double blind study with placebo control awaits.

There are many things that people use and say it works, but the known medications for hair loss is minoxidil which has defined properties. Rosmary Oil is an unknown entity but you can certainly try it and report back to us