Reverse split is intended to raise the Price Per Share (PPS) to make a stock seem more attractive, done by contracting the number of shares owned. IE You own a million shares at .01. They do a 10-to-1 reverse split, now you own 100,000 shares at 1.00. This is usually done by companies that are in trouble or that need more room to dilute without raising the A/S.

A/S is the amount of shares a company can issue total (this amount is controlled by the company and can be raised usually by a board of directors vote).

O/S is the number of shares held or issued.

A good resource that got me started as a beginner is investopedia.com