O'Sullivan, 41, chasing a fifth Welsh Open title, recovered from going a frame down to beat Tom Ford 4-1 and set up a meeting with Mark Davis.
Fellow Englishman Selby, 33, did not drop a frame as he beat Liam Highfield.
Wales' two-time world champion Mark Williams suffered a surprise 4-3 defeat by world number 115 Elliot Slessor.
There was another surprise exit as China's world number five Ding Junhui was knocked out in the first round in a 4-2 loss to Finland's Robin Hull.
World number four Judd Trump eased through 4-1 against Andrew Higginson, while Scottish Open champion Marco Fu beat Martin Gould 4-2.
Fifteen-year-old Welsh schoolboy Jackson Page is back in action on Wednesday, when he faces John Astley in the second round.
The teenage wildcard entry eliminated world number 123 Jason Weston in the first round of his debut professional tournament on Monday.
Find out how to get into snooker, pool and billiards with our fully inclusive guide.
Sign up to My Sport to follow snooker news and reports on the BBC app.

The Help to Buy Mortgage Guarantee Scheme allows lenders to offer loans to people offering a deposit as low as 5% thanks to government support.
The scheme is set to be closed at the end of the year.
Use of the scheme has declined, while mortgage lenders have reported that business in general is up sharply.
Lending to people with a 5% deposit, at a similar if not cheaper price, is available outside of the scheme.
In a letter to Chancellor Philip Hammond, the Bank's governor Mark Carney said that closing the scheme as planned would not cause lending to dry up.
"Given the decreasing usage of the scheme over time, the [Financial Policy] Committee judges that the closure of the scheme would be unlikely, in current market conditions, to affect significantly the provision of finance to prospective mortgagors, including high loan-to-value borrowers," he wrote.
The move is unlikely to affect other Help to Buy schemes, which include equity loans to buy newly-built homes, as well as the Help to Buy Isa savings scheme which is aimed at those saving for a deposit.
In its review of the Help to Buy Mortgage Guarantee Scheme, the Financial Policy Committee (FPC) said that the scheme accounted for 3% of total mortgage lending in the first quarter of the year. In 2014, it accounted for 6% of such lending.
It accounted for 25% of lending in the first three months of the year to those offering a deposit of less than 10% of the value of the home to be bought, compared with 70% in 2014.
There had been fears when launched that the scheme would create an unstable housing market bubble, but the FPC said that it had not posed a risk to financial stability.
It added that the scheme had not driven up prices in general, nor on the houses bought through the scheme.
There was a cap on loans which meant borrowers were only allowed up to 4.5 times their income. This, said the committee, had prevented a long list of highly indebted borrowers from building up.
The health of the mortgage market was stronger in August, separate figures from lenders have revealed.
Mohammad Jamei, senior economist for the Council of Mortgage Lenders (CML), said: "Widely voiced fears in recent months about the housing market have proved to be wide of the mark."
He said that there had been a post-referendum lull, but lending had rebounded in August, partly as a result of stimulus from the Bank of England - including a cut in interest rates.
The CML said that gross mortgage lending in the UK totalled Â£22.5bn in August. This was 7% higher than the previous month and 15% higher than the same month a year earlier.
It was the highest level of lending during any August since 2007, the CML said.
Where can I afford to live?

It is understood the plane came down at the privately-owned Peterborough Business Airport at about 11:50 GMT.
The pilot, a man aged in his 40s, was taken from the airfield to Peterborough City Hospital with a serious leg injury.
An air ambulance and other emergency vehicles were sent to the scene.
Defending champion Ronnie O'Sullivan and world number one Mark Selby both progressed to the second round of the Welsh Open on Tuesday.
[NEXT_CONCEPT]
A major scheme designed to prop up the UK housing market is no longer needed, the Bank of England has told the government.
[NEXT_CONCEPT]
A pilot has been injured in a light aircraft crash.