user54311: I am trying to write some data to SD card and read it back to serial monitor as well as display it to the OLED.
Both the sd card and oled work separately but they seem to be interfering with each other when combined. I have used Arduino SD and Adafruit OLED libraries.
Connections from Arduino Uno to Micro SD card module-
5V to SD VCC
GND TO SD GND
PIN 10 TO SD Chip Select
PIN 11 TO SD MOSI
PIN 12 TO SD MISO
PIN 13 TO SD SCK

Connections to OLED-
3.3V to OLED VCC
GND TO OLED GND
A4 TO OLED SDA
A5 TO OLED SCK

Here is the code -
#include <SPI.h>
#include <SD.h>

File myFile;

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels

// Declaration for an SSD1306 display connected to I2C (SDA, SCL pins)
#define OLED_RESET     -1 
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  while(!Serial) {
    ;
  }

  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { 
    Serial.println(F("SSD1306 allocation failed"));
    for(;;); // CODE GETS STUCK HERE. DISPLAY NEVER INITIALISES
  }     
  display.clearDisplay();           
  display.setTextSize(1);             
  display.setTextColor(WHITE);       
  display.setCursor(29,29);
  display.print("INITIALISING");
  display.display();
  delay(5000);

  if (!SD.begin(10)) {
    Serial.println("initialization failed!");
    while (1);
  }
  Serial.println("initialization done.");

  myFile = SD.open("test.txt", FILE_WRITE);

  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.println("testing 1, 2, 3.");
    // close the file:
    myFile.close();
    Serial.println("done.");
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }

  display.clearDisplay();

  myFile = SD.open("test.txt");
  if (myFile) {
    Serial.println("test.txt:");
    while (myFile.available()) {
      Serial.write(myFile.read());
      display.setCursor(0,0);
      display.print(myFile.read());
      display.display();
      delay(5000);
    }
    // close the file:
    myFile.close();
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }
}

void loop() {
  // put your main code here, to run repeatedly:

}

Code gets stuck at OLED initialization as mentiontioned above. If I replace these lines-
if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { 
    Serial.println(F("SSD1306 allocation failed"));
    for(;;); // CODE GETS STUCK HERE. DISPLAY NEVER INITIALISES
    } 

To this-
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);

I have run I2C scanner code on OLED so the address "0X3C" is correct.
The OLED still doesn't work and SD card initialises but arduino is writing wrong data to TXT file on SD card like this-
teóting 1,à2, ó®

Instead of -
testing 1, 2, 3.

I have also tried using U8G2 library's sketches with SD card in case Arduino was running out of RAM but it still doesn't work. I have also changed SD chip select to Arduino digital pin 4 but still same results.
On browsing and experimenting more,I found MISO OR MOSI PIN of SD maybe interfering with SDA/SCL pins of OLED. Maybe wiring needs to change.
ANY SUGGESTIONS???

user62075: A pullup resistor is needed for I2C, and adding them might fix some errors.

54311: <p>I am trying to write some data to SD card and read it back to serial monitor as well as display it to the OLED.</p>

<p>Both the sd card and oled work separately but they seem to be interfering with each other when combined. I have used Arduino SD and Adafruit OLED libraries.</p>

<p>Connections from Arduino Uno to Micro SD card module-</p>

<pre><code>5V to SD VCC
GND TO SD GND
PIN 10 TO SD Chip Select
PIN 11 TO SD MOSI
PIN 12 TO SD MISO
PIN 13 TO SD SCK
</code></pre>

<p>Connections to OLED-</p>

<pre><code>3.3V to OLED VCC
GND TO OLED GND
A4 TO OLED SDA
A5 TO OLED SCK
</code></pre>

<p>Here is the code -</p>

<pre><code>#include &lt;SPI.h&gt;
#include &lt;SD.h&gt;

File myFile;

#include &lt;Wire.h&gt;
#include &lt;Adafruit_GFX.h&gt;
#include &lt;Adafruit_SSD1306.h&gt;

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels

// Declaration for an SSD1306 display connected to I2C (SDA, SCL pins)
#define OLED_RESET     -1 
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &amp;Wire, OLED_RESET);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  while(!Serial) {
    ;
  }

  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { 
    Serial.println(F("SSD1306 allocation failed"));
    for(;;); // CODE GETS STUCK HERE. DISPLAY NEVER INITIALISES
  }     
  display.clearDisplay();           
  display.setTextSize(1);             
  display.setTextColor(WHITE);       
  display.setCursor(29,29);
  display.print("INITIALISING");
  display.display();
  delay(5000);

  if (!SD.begin(10)) {
    Serial.println("initialization failed!");
    while (1);
  }
  Serial.println("initialization done.");

  myFile = SD.open("test.txt", FILE_WRITE);

  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.println("testing 1, 2, 3.");
    // close the file:
    myFile.close();
    Serial.println("done.");
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }

  display.clearDisplay();

  myFile = SD.open("test.txt");
  if (myFile) {
    Serial.println("test.txt:");
    while (myFile.available()) {
      Serial.write(myFile.read());
      display.setCursor(0,0);
      display.print(myFile.read());
      display.display();
      delay(5000);
    }
    // close the file:
    myFile.close();
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }
}

void loop() {
  // put your main code here, to run repeatedly:

}
</code></pre>

<p>Code gets stuck at OLED initialization as mentiontioned above. If I replace these lines-</p>

<pre><code>if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { 
    Serial.println(F("SSD1306 allocation failed"));
    for(;;); // CODE GETS STUCK HERE. DISPLAY NEVER INITIALISES
    } 
</code></pre>

<p>To this-</p>

<pre><code>display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
</code></pre>

<p>I have run I2C scanner code on OLED so the address "0X3C" is correct.</p>

<p>The OLED still doesn't work and SD card initialises but arduino is writing wrong data to TXT file on SD card like this-</p>

<pre><code>teóting 1,à2, ó®
</code></pre>

<p>Instead of -</p>

<pre><code>testing 1, 2, 3.
</code></pre>

<p>I have also tried using U8G2 library's sketches with SD card in case Arduino was running out of RAM but it still doesn't work. I have also changed SD chip select to Arduino digital pin 4 but still same results.</p>

<p>On browsing and experimenting more,I found MISO OR MOSI PIN of SD maybe interfering with SDA/SCL pins of OLED. Maybe wiring needs to change.</p>

<p><strong>ANY SUGGESTIONS???</strong></p>
 62075: <p>A pullup resistor is needed for I2C, and adding them might fix some errors.</p>
