Discs played in the corners can't be flipped. If you can't take a corner, you can reduce its effectiveness by taking the squares adjacent to it.
If your opponent places his piece in the spot that is directly adjacent and diagonal to the corner and it's in the beginning or middle of the game, all you need is for any piece in that diagonal line to be your color for you to jump onto the corner.
You protect the corners by avoiding the "X" squares. These are the squares surrounding a corner.
Keep balanced edges
The next most important squares to control after the corners and the spaces adjacent to them are the edges of the board.
The edge rows can always be used to outflank the inner rows.
An unbalanced edge can be attacked by a skilful player who will give you a corner in order to take the other corner and the whole edge in between.
Moves that you can play but which your opponent cannot touch should be saved to the very last.
Try to control the diagonals at the endgame. This will allow you to play an X square without losing the corner.
Keep it Quiet in the beginning
In the early part of the game, avoid flipping too many pieces.
This is the secret of playing better Othello. Loud moves (moves that flip many discs) allow many options to your opponent for the next move.
Try to stay in the center and keep your pieces surrounded by your opponents.
Isolated C-squares can be trouble
C-squares (B1, G1, A2, H2, A7, H7, B8, and G8) are located on the edges and are next to corners. Placing a disc there without any disc next to it can spell trouble for a beginner as that position can be attacked by a more skilled player.
Keeping your pieces grouped together gives your opponent less options.
Avoid "edge creeping"
Just playing the edges might run your beginner or intermediate player out of moves, but you're just playing right into the hands of the advanced player.
Walls are long, unbroken lines of discs of one color sealing one side of the board.
If you are forced to make a bad move late into the game, it is usually a good idea to play in an area where an odd number of empty squares are left.
Occupying the least amount of cells at the beginning of the game leaves you with more options and your opponents fewer selections to make.