For the tax year of 2018, you might be likely to file as a nonresident on Form 1040NR.
Here is why:
As a F-1 visa holder, you are exempt (not counting days) for five years from the first year you enter the US. In your situation, you are considered as a nonresident for 2014-2018. As you change your status to an H-1B visa in October 2018, which is not exempt, IRS allows you to start counting days from October 2018. Technically, you are considered as a non-resident from January 1st - October 2018 while you are still on the F-visa. From October-December 2018 when you are on the H-visa, and since you do not stay more than 183 days from October until the end of the year, you are considered as a nonresident alien for the whole year.
You have another option. If you anticipate you will be staying in the US for at least 183 days in the year of 2019 or meet the Substantial Presence Test SPT, you can make an election to treat yourself as a resident from October 2018, to file as a dual-alien alien. Please see FirstYearChoice- Residency Starting Date under the First-Year Choice. If you meet the requirements, for the year of 2018, you will be filing as a nonresident for the first 10 months and a resident for the rest of the year.
Hope it helps!
