In plain words
Among the great seers I am Bhrigu; among words I am the single syllable; among yajñas I am japa-yajña; among immovable things I am the Himalaya.
Word for word
Among the great seers I [am] Bhṛgu; among utterances I am the one syllable; among yajñas I am the yajña of japa; among the immovable, the Himālaya.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
महर्षीणां भृगुः अहम् । गिरां वाचां पदलक्षणानाम् एकम् अक्षरम् ओङ्कारः अस्मि । यज्ञानां जपयज्ञः अस्मि, स्थावराणां स्थितिमतां हिमालयः ॥ २५ ॥
In plain words
Among the great seers, I am Bhṛgu. Among utterances — speech, that is, made of formed words — I am the single syllable: Oṃ. Among sacrifices I am the sacrifice of quiet repetition; and among all that is immovable, all that simply stands and stays, I am the Himālaya. || 25 ||
Close to the Sanskrit
Of the great seers I am Bhṛgu. Of utterances — that is, of words which have padas for their mark — I am the one syllable, Oṃ. Of sacrifices I am the sacrifice of repetition; of the immovable, that is, of those that possess fixity, [I am] the Himālaya. || 25 ||