<?php

$str = 'hdj 20k OR hjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl OR hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl OR hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl OR kdjkdjkdjkj OR djdldkkldkkkldklddklkldkl OR jdkdkjdjkjkdjkdjkdjkjkjkjkdkjkdjkjkdjkjk OR hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk';

print_r(preg_split('~(?<!\b20k\b) OR\b~', $str, 10));

Array
(
    [0] => hdj 20k OR hjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl
    [1] =>  hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl
    [2] =>  hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl
    [3] =>  kdjkdjkdjkj
    [4] =>  djdldkkldkkkldklddklkldkl
    [5] =>  jdkdkjdjkjkdjkdjkdjkjkjkjkdkjkdjkjkdjkjk
    [6] =>  hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl hdjhjdhjdhdhjdhjhdldkklkdklkllkldkldldkl kldkldkl
    [7] =>  llldlkdl dkld ldk
    [8] =>  llldlkdl dkld ldk
    [9] =>  llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk OR llldlkdl dkld ldk
)