Please cite the use of the OMNI and COHO datasets (adding the date of access) using DOI:

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI Hourly Data" {Data Set], 
NASA Space Physics Data Facility, https://doi.org/10.48322/1shr-ht18, Accessed on (DATE).

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI Combined Heliospheric 
Observations (COHO), Merged Magnetic Field, Plasma and Ephemeris, Definitive 
Hourly Data" [Data set], NASA Space Physics Data Facility, 
https://doi.org/10.48322/6ffx-3441, Accessed on .

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI 5-min Data" [Data set], 
NASA Space Physics Data Facility, https://doi.org/10.48322/gbpg-5r77, Accessed on (DATE)

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI 27-Day Data" [Data set], 
NASA Space Physics Data Facility, https://doi.org/10.48322/nmh3-jf75, Accessed on (DATE)

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI 1-min Data" [Data set], 
NASA Space Physics Data Facility, https://doi.org/10.48322/45bb-8792, Accessed on (DATE)

Papitashvili, Natalia E. and King, Joseph H. (2020), "OMNI Daily Data" [Data set], 
NASA Space Physics Data Facility,  https://doi.org/10.48322/5fmx-hv56, Accessed on (DATE)

Other Citations to use of OMNI data should be made inline in the body of text when first appropriate and/or in an Acknowledgement section.

Examples of the first would be "...[mention of parameters used] as extracted from NASA/GSFC's OMNI data set through OMNIWeb..."

In an Acknowledgement section: "We acknowledge use of NASA/GSFC's Space Physics Data Facility's OMNIWeb (or CDAWeb or ftp) service, and OMNI data."

If you have used data from few sources within OMNI, you should also cite the Principal Investigators who provided those data to OMNI and who are identified in the OMNIWeb-accessible OMNI documentation files. See
https://omniweb.gsfc.nasa.gov/html/ow_data.html or
https://omniweb.gsfc.nasa.gov/html/HROdocum.html

Specifically for use of OMNI data at hourly or lower resolution, you might cite from your text:

King, J.H. and N.E. Papitashvili, Solar wind spatial scales in and comparisons of hourly Wind and ACE plasma and magnetic field data, J. Geophys. Res., 110, A02104, 2005.
http://dx.doi.org/10.1029/2004JA010649