Update

I understood from you that you are using the API as follows:

apiClient.getJSON().setOffsetDateTimeFormat(DateTimeFormatter); 

and you do not have a parameter to pass timezone information. In this case, you can use DateTimeFormatter#withZone as shown below:

import java.time.OffsetDateTime;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuuMMddHHmmss", Locale.ENGLISH)
                                        .withZone(ZoneOffset.UTC);

        OffsetDateTime odt = OffsetDateTime.parse("20210817132649", formatter);

        System.out.println(odt);
    }
}

Output:

2021-08-17T13:26:49Z

ONLINE DEMO

i.e. now, your call will be:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuuMMddHHmmss", Locale.ENGLISH)
                                .withZone(ZoneOffset.UTC);

apiClient.getJSON().setOffsetDateTimeFormat(formatter); 

Original answer

Your date-time string does not have a timezone offset. Parse it into a LocalDateTime and then apply the offset to it.

Demo:

import java.time.LocalDateTime;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuuMMddHHmmss", Locale.ENGLISH);
        
        OffsetDateTime odt = LocalDateTime.parse("20210817132649", formatter)
                                .atOffset(ZoneOffset.UTC);
        
        System.out.println(odt);
    }
}

Output:

2021-08-17T13:26:49Z

ONLINE DEMO

The Z in the output is the timezone designator for zero-timezone offset. It stands for Zulu and specifies the Etc/UTC timezone (which has the timezone offset of +00:00 hours). You can specify a different timezone offset (e.g. ZoneOffset.of("+05:30")) as per your requirement.

In case you have ZoneId available

If you have ZoneId available, you should parse the given date-time string into a LocalDateTime and then apply the ZoneId to it to get a ZonedDateTime from which you can always obtain an OffsetDateTime. The best thing about a ZonedDateTime is that it has been designed to adjust the timezone offset automatically whereas an OffsetDateTime is used to represent a fixed timezone offset.

Demo:

import java.time.LocalDateTime;
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuuMMddHHmmss", Locale.ENGLISH);

        ZonedDateTime zdt = LocalDateTime.parse("20210817132649", formatter)
                                .atZone(ZoneId.of("Etc/UTC"));

        OffsetDateTime odt = zdt.toOffsetDateTime();

        System.out.println(odt);
    }
}

Output:

2021-08-17T13:26:49Z

ONLINE DEMO

You can specify a different ZoneId(e.g. ZoneId.of("Asia/Kolkata")) as per your requirement.

Learn more about the modern Date-Time API^* from Trail: Date Time.

^{* If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring. Note that Android 8.0 Oreo already provides support for java.time.}

It should be

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");

//or

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSz");

instead of

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-ddTHH:mm:ss.SSSZ");

From JAVADoc:

Offset X and x: This formats the offset based on the number of pattern letters. One letter outputs just the hour, such as '+01', unless the minute is non-zero in which case the minute is also output, such as '+0130'. Two letters outputs the hour and minute, without a colon, such as '+0130'. Three letters outputs the hour and minute, with a colon, such as '+01:30'. Four letters outputs the hour and minute and optional second, without a colon, such as '+013015'. Five letters outputs the hour and minute and optional second, with a colon, such as '+01:30:15'. Six or more letters throws IllegalArgumentException. Pattern letter 'X' (upper case) will output 'Z' when the offset to be output would be zero, whereas pattern letter 'x' (lower case) will output '+00', '+0000', or '+00:00'.