Background: South Africa's PIRLS 2021 outcomes revealed that learners struggle with comprehension. The reasons for poor reading need to be identified and addressed to stem this challenge. Aim: This study assessed Sesotho Grade 4 learners' knowledge of simple and complex letter sounds, their oral reading fluency and the relationship to their oral reading comprehension. Setting: Data were collected by testing 103 Grade 4 Sesotho home language learners in five schools. Method: Early Grade Reading Assessment instruments adapted for Sesotho were used, which included simple and complex letter sound knowledge tests and an oral reading fluency test with six comprehension questions. Results: Firstly, the results show that most of the sampled Grade 4 Sesotho learners could identify only 33.01 of the 70 single-letter sounds and 19.52 of 40 complex letter sounds. Secondly, most learners struggled to correctly pronounce single-letter and single-syllable function words with simple letter sounds. Thus, learners could only read 30.54 words correct per minute (wcpm). Lastly, significant positive correlations between complex letter sounds like diagraphs and trigraphs, reading accuracy and oral reading comprehension were revealed. Conclusion: Knowledge of letter sounds is vital for decoding alphabetic languages with shallow orthographies, which follow the phonological route for processing written texts. It facilitates accurate decoding, which is a prerequisite for reading fluency and comprehension. Contribution: There is a need for Sesotho evidence-based and adequately paced phonics programmes to systematically teach the knowledge of letter sounds in the foundation phase to strengthen reading fluency for comprehension.