Gāyatrī mantra should have two ॐ; one at the beginning of vyāhṛti-s and another for the second part, the Vedic mantra.

ॐ भूर्भुवस्सुवः।
ॐ तत्सवितुर्वरेण्यम्।
भर्गो देवस्य धीमहि।
धियो यो नः प्रचोदयात् ॥

om bhūrbhuvassuvaḥ |
om tatsaviturvareṇyam |
bhargo devasya dhīmahi |
dhiyo yo naḥ pracodayāt ||