what to do?
I bought 100 shares of apple stock at 138 and wrote a call option (MAY 150).
Now the stock is going up more than I Thought.  What is the best strategy now, wait for the option to expire?
What should I expect if the option is exercised?

Congratulations on a winning trade.

There is never a single, best, strategy that works for every investor.

You are currently in an excellent situation, but please remember that May expiration is still a long time away (6 weeks) and the calls
are barely in the money (AAPL is 153 as I write this).  It's far too early to worry about what may happen at May expiration.

1) If the call is eventually exercised, consider that a great result.  It represents the best possible outcome when you own a covered call position
because it provides the maximum possible profit for the trade.  You would sell your stock at $150 per share and keep the option premium you collected.

2) You currently have a nice profit because the stock has increased in value by much more than the option.  If you are nervous about losing that profit, you can close the position by buying the call and selling the stock. Only you know if you want to hold this position longer, seeking an ever better profit.

3) If you hold longer and if the stock does not decline below the strike price, you will make much more money than you already earned.  The trouble with hoping for this outcome is that the company is going to announce its earnings results in less than 3 weeks, and that could result in a big change (up or down) in the stock price.  Obviously up is good for you and down is not so good.  If you expect good news, hold.  If you are worried about bad news, you should probably close before news is announced.

4) Thus, the 'best (for you)' strategy now is doing what makes you the most comfortable:
  • Take your profit
  • Hold, hoping the stock remains at its current level, or higher
  • Wait and then reconsider your choices before earnings news (Apr 23)

Mark D. Wolfinger
The Rookie's Guide to Options:
The Beginner's Handbook of Trading Equity Options