TicTacToe in OOP

I'm trying to learn some OOP programming using Java, so I've created TicTacToe. Could you give me some advice on how I can improve this code?

Player.java:

package com.kamil.dude.tactoe;

import java.util.Random;
import java.util.Scanner;

public class Player {
public char currentPlayer;

public Player() {
    Random generator = new Random();
    currentPlayer = (generator.nextBoolean()) == true ? 'O' : 'X';
}

public void setPlayer() {
    currentPlayer = (currentPlayer == 'O') ? 'X' : 'O';
}

public int getInput(int size) {
    Scanner in = new Scanner(System.in);
    int fieldNumber = in.nextInt();
    while (fieldNumber < 1 || fieldNumber > size) {
        fieldNumber = in.nextInt();
    }
    return fieldNumber;
}
}

GameBoard.java:

package com.kamil.dude.tactoe;

public class GameBoard {
private Player player;
private char gameBoard[]; // I want to represent gameBoard as an array of buttons
private int fieldNumber;
private int filledFieldsNumber;
private final int lengthOfGameBoardSide;
private final int bottomLeftCornerIndex; // helps to check slash lines
private int size;

public GameBoard(int size) {
    this.lengthOfGameBoardSide = (int) Math.sqrt(size);
    this.bottomLeftCornerIndex = size - ((int) Math.sqrt(size) - 1);
    this.size = size;
    player = new Player();
    gameBoard = new char[size];
    for (int i = 0; i < size; ++i) {
        gameBoard[i] = '0';
    }
}

public void draw() {
    for (int i = 0, charsInRow = 0; i < size; ++i, ++charsInRow) {
        if (charsInRow == lengthOfGameBoardSide) {
            charsInRow = 0;
            System.out.println();
        }
        if (gameBoard[i] == '0') {
            if (i < 9)
                System.out.print(i + 1 + "  ");
            else
                System.out.print(i + 1 + " ");
        }
        else if (gameBoard[i] == 'O')
            System.out.print("O  ");
        else
            System.out.print("X  ");
    }
    System.out.println();
}

public void checkInput() {
    fieldNumber = player.getInput(size);
    if (player.currentPlayer == 'O' && isFieldFree(fieldNumber)) {
        gameBoard[fieldNumber - 1] = 'O';
        ++filledFieldsNumber;
        player.setPlayer();
    }   
    else if (player.currentPlayer == 'X' && isFieldFree(fieldNumber)) {
        gameBoard[fieldNumber - 1] = 'X';
        ++filledFieldsNumber;
        player.setPlayer();
    }   
}

private boolean isFieldFree(int fieldNumber) {
    return (gameBoard[fieldNumber - 1] == '0') ? true : false;
}

public boolean isGameEnded() {
    if (checkIfWin() || checkIfDraw())
        return true;
    return false;
}

private boolean checkIfWin() {
    if (checkVerticalLines() || checkHorizontalLines() || checkSlashLines())
        return true;
    return false;
}

private boolean checkVerticalLines() {
    int repeatNumber = 0;
    for (int i = 0; i < lengthOfGameBoardSide; ++i) {
        for (int j = i; j < size && repeatNumber != lengthOfGameBoardSide - 1; j += lengthOfGameBoardSide) {
            if (gameBoard[j] != '0' && gameBoard[j + lengthOfGameBoardSide] == gameBoard[j]) {
                ++repeatNumber;
            } else {
                repeatNumber = 0;
                break;
            }
        }
        if (checkRepeatedSignsNumber(repeatNumber)) 
            return true;
    }
    return false;
}

private boolean checkHorizontalLines() {
    int repeatNumber = 0;
    for (int i = 0; i < bottomLeftCornerIndex; i += lengthOfGameBoardSide) {
        for (int j = i; j < size && repeatNumber != lengthOfGameBoardSide - 1; ++j) {
            if (gameBoard[j] != '0' && gameBoard[j + 1] == gameBoard[j]) {
                ++repeatNumber;
            } else {
                repeatNumber = 0;
                break;
            }
        }
        if (checkRepeatedSignsNumber(repeatNumber)) 
            return true;
    }
    return false;
}

private boolean checkSlashLines() {
    if (checkBackSlash() || checkSlash())
        return true;
    return false;
}

private boolean checkBackSlash() {
    int repeatNumber = 0;
    for (int i = 0; i < size && repeatNumber != lengthOfGameBoardSide - 1; i += lengthOfGameBoardSide + 1) {
        if (gameBoard[i] != '0' && gameBoard[i + (lengthOfGameBoardSide + 1)] == gameBoard[i]) {
            ++repeatNumber;
        } else {
            repeatNumber = 0;
            break;
        }
        if (checkRepeatedSignsNumber(repeatNumber)) 
            return true;
    }
    return false;
}

private boolean checkSlash() {
    int repeatNumber = 0;
    for (int i = 2; i < bottomLeftCornerIndex && repeatNumber != lengthOfGameBoardSide - 1; i += size - bottomLeftCornerIndex) {
        if (gameBoard[i] != '0' && gameBoard[i + (size - bottomLeftCornerIndex)] == gameBoard[i]) {
            ++repeatNumber;
        } else {
            repeatNumber = 0;
            break;
        }
        if (checkRepeatedSignsNumber(repeatNumber)) 
            return true;
    }
    return false;
}

private boolean checkRepeatedSignsNumber(int repeatNumber) {
    if (repeatNumber == lengthOfGameBoardSide - 1) {
        draw();
        player.setPlayer();
        System.out.println("Player " + player.currentPlayer + " wins");
        return true;
    }
    return false;
}

private boolean checkIfDraw() {
    if (filledFieldsNumber == size) {
        draw();
        System.out.println("Draw!");
        return true;
    }
    return false;
}
}

Main.java:

package com.kamil.dude.tactoe;

public class Main {

public static void main(String[] args) {
    GameBoard board = new GameBoard(9); // You can change game board size by giving number of fields
    for(;;) {
        board.draw();
        board.checkInput(); 
        if(board.isGameEnded() == true)
            break;
    }
}
}